Make language option `GNUAsm` discoverable with `__has_extension` macro.

[vsapsai]

Summary

This can be used for better support of -fno-gnu-inline-asm builds.

rdar://problem/49540880

[vsapsai]

Ping.

[aaron.ballman]

LGTM!

[vsapsai]

Thanks for the review, Aaron.

include/clang/Basic/Features.def

 // C++14 features supported by other languages as extensions.
 EXTENSION(cxx_binary_literals, true)
 EXTENSION(cxx_init_captures, LangOpts.CPlusPlus11)
 EXTENSION(cxx_variable_templates, LangOpts.CPlusPlus)
 // Miscellaneous language extensions
 EXTENSION(overloadable_unmarked, true)
 EXTENSION(pragma_clang_attribute_namespaces, true)
 EXTENSION(pragma_clang_attribute_external_declaration, true)
+EXTENSION(gnu_asm, LangOpts.GNUAsm)
 
 #undef EXTENSION
 #undef FEATURE

test/Parser/asm.c

 // RUN: %clang_cc1 -fsyntax-only -verify %s
 
+#if !__has_extension(gnu_asm)
+#error Extension 'gnu_asm' should be available by default
+#endif
+
 void f1() {
   // PR7673: Some versions of GCC support an empty clobbers section.
   asm ("ret" : : :);
 }
 
 void f2() {
   asm("foo" : "=r" (a)); // expected-error {{use of undeclared identifier 'a'}}
   asm("foo" : : "r" (b)); // expected-error {{use of undeclared identifier 'b'}} 

test/Parser/no-gnu-inline-asm.c

 // RUN: %clang_cc1 %s -triple i686-apple-darwin -verify -fsyntax-only -fno-gnu-inline-asm
 
+#if __has_extension(gnu_asm)
+#error Expected extension 'gnu_asm' to be disabled
+#endif
+
 asm ("INST r1, 0"); // expected-error {{GNU-style inline assembly is disabled}}
 
 void foo() __asm("__foo_func"); // AsmLabel is OK
 int foo1 asm("bar1") = 0; // OK
 
 asm(" "); // Whitespace is OK
 
 void f (void) {
   long long foo = 0, bar;
   asm volatile("INST %0, %1" : "=r"(foo) : "r"(bar)); // expected-error {{GNU-style inline assembly is disabled}}
   asm (""); // Empty is OK
   return;
 }