[libc] Update preprocessor definitions for type detection

[gchatelet]

This is to track the items in #81870 (comment)

i.e.,

• [libc][NFC] rename float.h macro file to types.h #83190
• Rename all #define in this file to LIBC_TYPES_HAS_...
  [libc] Provide LIBC_TYPES_HAS_INT64 #83441
  [libc] Provide LIBC_TYPES_HAS_INT128 #84149
  [libc][NFC] Rename LIBC_LONG_DOUBLE_IS_ macros #83399
  [libc][NFC] Rename LIBC_COMPILER_HAS_C23_FLOAT16 to LIBC_TYPES_HAS_FLOAT16 #83396
  [libc][NFC] Rename LIBC_COMPILER_HAS_FLOAT128 to LIBC_TYPES_HAS_FLOAT128 #83395
• [libc][NFC] Don't expose how float16 and float128 types are provided #83818 (i.e. no LIBC_COMPILER_HAS_FLOAT128_EXTENSION nor LIBC_COMPILER_HAS_C23_FLOAT128). Added benefit of not defining LIBC_COMPILER_... outside of the compiler.h file.
• Update the documentation

[gchatelet]

FYI @nickdesaulniers

[llvmbot]

@llvm/issue-subscribers-libc

Author: Guillaume Chatelet (gchatelet)

This is to track the items in https://github.com//pull/81870#issuecomment-1948099726

i.e.,

• Rename float.h into types.h
• Rename all #define in this file to LIBC_TYPES_HAS_...
LIBC_TYPES_HAS_INT64
LIBC_TYPES_HAS_FLOAT16
LIBC_TYPES_HAS_FLOAT128
• Don't expose how the type is provided and use float16 or float128 solely (i.e. no LIBC_COMPILER_HAS_FLOAT128_EXTENSION nor LIBC_COMPILER_HAS_C23_FLOAT128). Added benefit of not defining LIBC_COMPILER_... outside of the compiler.h file.
• Update the documentation