[CodeGen] fix inline builtin-related breakage from D78162

In cases where we have multiple decls of an inline builtin, we may need
to go hunting for the one with a definition when setting function
attributes.

An additional test-case was provided on
ClangBuiltLinux/linux#979

(cherry picked from commit 9490808)

clang/lib/CodeGen/CodeGenModule.cpp

@@ -1848,9 +1848,15 @@ void CodeGenModule::SetFunctionAttributes(GlobalDecl GD, llvm::Function *F,
      F->setSection(SA->getName());
 
   // If we plan on emitting this inline builtin, we can't treat it as a builtin.
-  if (FD->isInlineBuiltinDeclaration() && shouldEmitFunction(FD)) {
-    F->addAttribute(llvm::AttributeList::FunctionIndex,
-                    llvm::Attribute::NoBuiltin);
+  if (FD->isInlineBuiltinDeclaration()) {
+    const FunctionDecl *FDBody;
+    bool HasBody = FD->hasBody(FDBody);
+    (void)HasBody;
+    assert(HasBody && "Inline builtin declarations should always have an "
+                      "available body!");
+    if (shouldEmitFunction(FDBody))
+      F->addAttribute(llvm::AttributeList::FunctionIndex,
+                      llvm::Attribute::NoBuiltin);
   }
 
   if (FD->isReplaceableGlobalAllocationFunction()) {

clang/test/CodeGen/memcpy-no-nobuiltin-if-not-emitted.cpp

@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 -triple i686-linux-gnu -std=c++11 -S -emit-llvm -o - %s | FileCheck %s
+//
+// Regression test for the issue reported at
+// https://reviews.llvm.org/D78162#1986104
+
+typedef unsigned long size_t;
+
+extern "C" __inline__ __attribute__((__gnu_inline__)) void *memcpy(void *a, const void *b, unsigned c) {
+  return __builtin_memcpy(a, b, c);
+}
+void *memcpy(void *, const void *, unsigned);
+
+// CHECK-LABEL: define void @_Z1av
+void a() { (void)memcpy; }
+
+// CHECK-NOT: nobuiltin