[C++20] [Modules] Don't emit function bodies which is noinline and availabl externally

A workaround for llvm#60996

As the title suggested, we can avoid emitting available externally
functions which is marked as noinline already. Such functions should
contribute nothing for optimizations.

The update for docs will be sent seperately if this god approved.

Author: ChuanqiXu9

clang/lib/CodeGen/CodeGenModule.cpp

@@ -3847,6 +3847,9 @@ bool CodeGenModule::shouldEmitFunction(GlobalDecl GD) {
   if (CodeGenOpts.OptimizationLevel == 0 && !F->hasAttr<AlwaysInlineAttr>())
     return false;
 
+  if (F->hasAttr<NoInlineAttr>())
+    return false;
+
   if (F->hasAttr<DLLImportAttr>() && !F->hasAttr<AlwaysInlineAttr>()) {
     // Check whether it would be safe to inline this dllimport function.
     DLLImportFunctionVisitor Visitor;

clang/test/Modules/no-import-func-body.cppm

@@ -0,0 +1,45 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: cd %t
+//
+// RUN: %clang_cc1 -std=c++20 -O1 -triple %itanium_abi_triple %t/a.cppm \
+// RUN:     -emit-module-interface -o %t/a.pcm
+// RUN: %clang_cc1 -std=c++20 -O1 -triple %itanium_abi_triple %t/b.cppm \
+// RUN:     -emit-module-interface -fprebuilt-module-path=%t -o %t/b.pcm
+// RUN: %clang_cc1 -std=c++20 -O1 -triple %itanium_abi_triple %t/c.cppm \
+// RUN:     -emit-module-interface -fprebuilt-module-path=%t -o %t/c.pcm
+// RUN: %clang_cc1 -std=c++20 -O1 -triple %itanium_abi_triple %t/c.pcm -S \
+// RUN:     -emit-llvm -disable-llvm-passes -o - | FileCheck %t/c.cppm
+
+//--- a.cppm
+export module a;
+export int a() {
+    return 43;
+}
+export __attribute__((noinline)) int a_noinline() {
+    return 44;
+}
+
+//--- b.cppm
+export module b;
+export import a;
+export int b() {
+    return 43 + a();
+}
+
+export __attribute__((noinline)) int b_noinline() {
+    return 43 + a();
+}
+
+//--- c.cppm
+export module c;
+export import b;
+export int c() {
+    return 43 + b() + a() + b_noinline() + a_noinline();
+}
+
+// CHECK: define{{.*}}available_externally{{.*}}@_ZW1b1bv(
+// CHECK: define{{.*}}available_externally{{.*}}@_ZW1a1av(
+
+// CHECK: declare{{.*}}@_ZW1b10b_noinlinev()
+// CHECK: declare{{.*}}@_ZW1a10a_noinlinev()