[CodeGen] only add nobuiltin to inline builtins if we'll emit them

There are some inline builtin definitions that we can't emit
(isTriviallyRecursive & callers go into why). Marking these
nobuiltin is only useful if we actually emit the body, so don't mark
these as such unless we _do_ plan on emitting that.

This suboptimality was encountered in Linux (see some discussion on
D71082, and ClangBuiltLinux/linux#979).

Differential Revision: https://reviews.llvm.org/D78162

clang/lib/CodeGen/CodeGenModule.cpp

@@ -1908,7 +1908,8 @@ void CodeGenModule::SetFunctionAttributes(GlobalDecl GD, llvm::Function *F,
   else if (const auto *SA = FD->getAttr<SectionAttr>())
      F->setSection(SA->getName());
 
-  if (FD->isInlineBuiltinDeclaration()) {
+  // 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);
   }

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

@@ -0,0 +1,25 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-unknown -S -emit-llvm -o - %s | FileCheck %s
+//
+// Verifies that clang doesn't mark an inline builtin definition as `nobuiltin`
+// if the builtin isn't emittable.
+
+typedef unsigned long size_t;
+
+// always_inline is used so clang will emit this body. Otherwise, we need >=
+// -O1.
+#define AVAILABLE_EXTERNALLY extern inline __attribute__((always_inline)) \
+    __attribute__((gnu_inline))
+
+AVAILABLE_EXTERNALLY void *memcpy(void *a, const void *b, size_t c) {
+  return __builtin_memcpy(a, b, c);
+}
+
+// CHECK-LABEL: define void @foo
+void foo(void *a, const void *b, size_t c) {
+  // Clang will always _emit_ this as memcpy. LLVM turns it into @llvm.memcpy
+  // later on if optimizations are enabled.
+  // CHECK: call i8* @memcpy
+  memcpy(a, b, c);
+}
+
+// CHECK-NOT: nobuiltin