[clang] Use the appropriate definition when checking FunctionDecl::isInlineBuiltinDeclaration

[serge-sans-paille]

Summary

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

[mstorsjo]

LGTM, thanks!

I'm not familiar with the code in question to know whether this really is the right thing to do etc, but this fixes my testcase (both the reduced one and the full one).

clang/lib/AST/Decl.cpp

bool FunctionDecl::isInlineBuiltinDeclaration() const {
   if (!hasBody(Definition))
     return false;
 
   if (!Definition->isInlineSpecified() ||
       !Definition->hasAttr<AlwaysInlineAttr>())
     return false;
 
   ASTContext &Context = getASTContext();
-  switch (Context.GetGVALinkageForFunction(this)) {
+  switch (Context.GetGVALinkageForFunction(Definition)) {
   case GVA_Internal:
   case GVA_DiscardableODR:
   case GVA_StrongODR:
     return false;
   case GVA_AvailableExternally:
   case GVA_StrongExternal:
     return true;
   }

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

+// 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