Revert "[NFC] Remove unnecessary 'Builtins.def' file."

This reverts commit 5a95378.

It was pointed out that this serves as documentation for the targets
where the builtin files have yet to be converted, so this should be left
in place.  Reverting!

clang/include/clang/Basic/Builtins.def

@@ -0,0 +1,102 @@
+//===--- Builtins.def - Builtin function info database ----------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// This is only documentation for the database layout. This will be removed once
+// all builtin databases are converted to tablegen files
+
+// The second value provided to the macro specifies the type of the function
+// (result value, then each argument) as follows:
+//  v -> void
+//  b -> boolean
+//  c -> char
+//  s -> short
+//  i -> int
+//  h -> half (__fp16, OpenCL)
+//  x -> half (_Float16)
+//  y -> half (__bf16)
+//  f -> float
+//  d -> double
+//  z -> size_t
+//  w -> wchar_t
+//  F -> constant CFString
+//  G -> id
+//  H -> SEL
+//  M -> struct objc_super
+//  a -> __builtin_va_list
+//  A -> "reference" to __builtin_va_list
+//  V -> Vector, followed by the number of elements and the base type.
+//  q -> Scalable vector, followed by the number of elements and the base type.
+//  Q -> target builtin type, followed by a character to distinguish the builtin type
+//    Qa -> AArch64 svcount_t builtin type.
+//  E -> ext_vector, followed by the number of elements and the base type.
+//  X -> _Complex, followed by the base type.
+//  Y -> ptrdiff_t
+//  P -> FILE
+//  J -> jmp_buf
+//  SJ -> sigjmp_buf
+//  K -> ucontext_t
+//  p -> pid_t
+//  . -> "...".  This may only occur at the end of the function list.
+//
+// Types may be prefixed with the following modifiers:
+//  L   -> long (e.g. Li for 'long int', Ld for 'long double')
+//  LL  -> long long (e.g. LLi for 'long long int', LLd for __float128)
+//  LLL -> __int128_t (e.g. LLLi)
+//  Z   -> int32_t (require a native 32-bit integer type on the target)
+//  W   -> int64_t (require a native 64-bit integer type on the target)
+//  N   -> 'int' size if target is LP64, 'L' otherwise.
+//  O   -> long for OpenCL targets, long long otherwise.
+//  S   -> signed
+//  U   -> unsigned
+//  I   -> Required to constant fold to an integer constant expression.
+//
+// Types may be postfixed with the following modifiers:
+// * -> pointer (optionally followed by an address space number, if no address
+//               space is specified than any address space will be accepted)
+// & -> reference (optionally followed by an address space number)
+// C -> const
+// D -> volatile
+// R -> restrict
+
+// The third value provided to the macro specifies information about attributes
+// of the function.  These must be kept in sync with the predicates in the
+// Builtin::Context class.  Currently we have:
+//  n -> nothrow
+//  r -> noreturn
+//  U -> pure
+//  c -> const
+//  t -> signature is meaningless, use custom typechecking
+//  T -> type is not important to semantic analysis and codegen; recognize as
+//       builtin even if type doesn't match signature, and don't warn if we
+//       can't be sure the type is right
+//  F -> this is a libc/libm function with a '__builtin_' prefix added.
+//  f -> this is a libc/libm function without a '__builtin_' prefix, or with
+//       'z', a C++ standard library function in namespace std::. This builtin
+//       is disableable by '-fno-builtin-foo' / '-fno-builtin-std-foo'.
+//  h -> this function requires a specific header or an explicit declaration.
+//  i -> this is a runtime library implemented function without the
+//       '__builtin_' prefix. It will be implemented in compiler-rt or libgcc.
+//  p:N: -> this is a printf-like function whose Nth argument is the format
+//          string.
+//  P:N: -> similar to the p:N: attribute, but the function is like vprintf
+//          in that it accepts its arguments as a va_list rather than
+//          through an ellipsis
+//  s:N: -> this is a scanf-like function whose Nth argument is the format
+//          string.
+//  S:N: -> similar to the s:N: attribute, but the function is like vscanf
+//          in that it accepts its arguments as a va_list rather than
+//          through an ellipsis
+//  e -> const, but only when -fno-math-errno and FP exceptions are ignored
+//  g -> const when FP exceptions are ignored
+//  j -> returns_twice (like setjmp)
+//  u -> arguments are not evaluated for their side-effects
+//  V:N: -> requires vectors of at least N bits to be legal
+//  C<N,M_0,...,M_k> -> callback behavior: argument N is called with argument
+//                      M_0, ..., M_k as payload
+//  z -> this is a function in (possibly-versioned) namespace std
+//  E -> this function can be constant evaluated by Clang frontend