[Clang][CodeGen] support alias attribute w/ gnu_inline

[nickdesaulniers]

Summary

r369705 did not consider the addition of gnu_inline on function
declarations of alias attributed functions. This resulted in a reported
regression in the clang-9-rc4 release from the Zig developers building
glibc, which was observable as a failed assertion:

llvm-project/clang/lib/AST/Decl.cpp:3336: bool
clang::FunctionDecl::isInlineDefinitionExternallyVisible() const:
Assertion `(doesThisDeclarationHaveABody() || willHaveBody()) && "Must
be a function definition"' failed.

Alias function declarations do not have bodies, so allow us to proceed
if we have the alias function attribute but no body/definition, and add
a test case. The emitted symbols and their linkage matches GCC for the
added test case.

Link: https://bugs.llvm.org/show_bug.cgi?id=43268

[andrewrk]

clang/lib/AST/Decl.cpp

3352

It looks like the rparen is in the wrong place. I'd expect:

assert((doesThisDeclarationHaveABody() || willHaveBody() ||
     hasAttr<AliasAttr>()) && "Must be a function definition");

[nickdesaulniers]

• adjust parens

clang/lib/AST/Decl.cpp

 /// inline definition becomes externally visible (C99 6.7.4p6).
 ///
 /// In GNU89 mode, or if the gnu_inline attribute is attached to the function
 /// definition, we use the GNU semantics for inline, which are nearly the
 /// opposite of C99 semantics. In particular, "inline" by itself will create
 /// an externally visible symbol, but "extern inline" will not create an
 /// externally visible symbol.
 bool FunctionDecl::isInlineDefinitionExternallyVisible() const {
-  assert((doesThisDeclarationHaveABody() || willHaveBody()) &&
-         "Must be a function definition");
+  assert((doesThisDeclarationHaveABody() || willHaveBody()) ||
+         hasAttr<AliasAttr>() && "Must be a function definition");
   assert(isInlined() && "Function must be inline");
   ASTContext &Context = getASTContext();
 
   if (Context.getLangOpts().GNUInline || hasAttr<GNUInlineAttr>()) {
     // Note: If you change the logic here, please change
     // doesDeclarationForceExternallyVisibleDefinition as well.
     //
     // If it's not the case that both 'inline' and 'extern' are

clang/test/CodeGen/alias.c

 // CHECKGLOBALS: @test10_foo = internal
 // CHECKGLOBALS-NOT: @test10_foo = dso_local
 int test10;
 static int test10_foo __attribute__((alias("test10")));
 // CHECKGLOBALS: @test11_foo = internal
 // CHECKGLOBALS-NOT: @test11_foo = dso_local
 void test11(void) {}
 static void test11_foo(void) __attribute__((alias("test11")));
+
+// Test that gnu_inline+alias work.
+// CHECKGLOBALS: @test12_alias = alias void (), void ()* @test12
+void test12(void) {}
+inline void test12_alias(void) __attribute__((gnu_inline, alias("test12")));