[clang] Use the appropriate definition when checking FunctionDecl::is…

…InlineBuiltinDeclaration

This is a follow-up to https://reviews.llvm.org/D148723 and fixes the
bug reported by @mstorsjo.

Differential Revision: https://reviews.llvm.org/D151783

clang/lib/AST/Decl.cpp

@@ -3322,7 +3322,7 @@ bool FunctionDecl::isInlineBuiltinDeclaration() const {
     return false;
 
   ASTContext &Context = getASTContext();
-  switch (Context.GetGVALinkageForFunction(this)) {
+  switch (Context.GetGVALinkageForFunction(Definition)) {
   case GVA_Internal:
   case GVA_DiscardableODR:
   case GVA_StrongODR:

clang/test/CodeGen/memcpy-inline-builtin-mutliple-decl.c

@@ -0,0 +1,29 @@
+// RUN: %clang_cc1 -triple i686-w64-mingw32 -S -emit-llvm -disable-llvm-passes -o - %s | FileCheck %s
+//
+// Verifies that clang detects memcpy inline version and uses it instead of the builtin.
+// Checks that clang correctly walks through multiple forward declaration.
+
+typedef unsigned int size_t;
+
+void *memcpy(void *_Dst, const void *_Src, size_t _Size);
+
+extern __inline__ __attribute__((__always_inline__, __gnu_inline__)) __attribute__((__artificial__))
+void *memcpy(void *__dst, const void *__src, size_t __n)
+{
+  return __builtin___memcpy_chk(__dst, __src, __n, __builtin_object_size((__dst), ((0) > 0) && (2 > 1)));
+}
+
+void *memcpy(void *_Dst, const void *_Src, size_t _Size);
+
+char *a, *b;
+void func(void) {
+    memcpy(a, b, 42);
+}
+
+// CHECK-LABEL: define {{.*}} @func(
+// CHECK: call ptr @memcpy.inline
+
+// CHECK-LABEL: declare {{.*}} @memcpy(
+
+// CHECK-LABEL: define {{.*}} @memcpy.inline(
+// CHECK: call ptr @__memcpy_chk