[WebAssembly] Setting export_name implies llvm.used

This change updates the clang front end to add symbols to llvm.used
when they have explicit export_name attribute.

Differential Revision: https://reviews.llvm.org/D71493

clang/lib/Sema/SemaDeclAttr.cpp

@@ -5772,8 +5772,8 @@ static void handleWebAssemblyExportNameAttr(Sema &S, Decl *D, const ParsedAttr &
   if (!S.checkStringLiteralArgumentAttr(AL, 0, Str, &ArgLoc))
     return;
 
-  FD->addAttr(::new (S.Context)
-                  WebAssemblyExportNameAttr(S.Context, AL, Str));
+  D->addAttr(::new (S.Context) WebAssemblyExportNameAttr(S.Context, AL, Str));
+  D->addAttr(UsedAttr::CreateImplicit(S.Context));
 }
 
 static void handleWebAssemblyImportModuleAttr(Sema &S, Decl *D, const ParsedAttr &AL) {

clang/test/CodeGen/wasm-export-name.c

@@ -6,6 +6,8 @@ int foo(void) {
   return 43;
 }
 
+// CHECK: @llvm.used = appending global [1 x i8*] [i8* bitcast (i32 ()* @foo to i8*)]
+
 // CHECK: define i32 @foo() [[A:#[0-9]+]]
 
 // CHECK: attributes [[A]] = {{{.*}} "wasm-export-name"="bar" {{.*}}}