Fix compiler_builtins.m test to not rely on including system stdlib.h and malloc.h

[hans]

Summary

Importing _Builtin_intrinsics.sse and avx would transitively pull in those
headers, and the test would fail when building in an environment where
they were not available on the include path.

This fixes PR20995 for me, and I'd like to get this onto the 3.6 branch.

[rsmith]

LGTM

test/Modules/Inputs/System/usr/include/stdlib.h

+typedef __SIZE_TYPE__ size_t;

test/Modules/compiler_builtins.m

 // RUN: rm -rf %t
-// RUN: %clang -fsyntax-only -fmodules -fmodules-cache-path=%t -D__need_wint_t %s -Xclang -verify
-// RUN: %clang -fsyntax-only -std=c99 -fmodules -fmodules-cache-path=%t -D__need_wint_t %s -Xclang -verify
+// RUN: %clang -fsyntax-only -fmodules -fmodules-cache-path=%t -D__need_wint_t %s -I%S/Inputs/System/usr/include -Xclang -verify
+// RUN: %clang -fsyntax-only -std=c99 -fmodules -fmodules-cache-path=%t -D__need_wint_t %s -I%S/Inputs/System/usr/include -Xclang -verify
 // expected-no-diagnostics
 
 #ifdef __SSE__
 @import _Builtin_intrinsics.intel.sse;
 #endif
 
 #ifdef __AVX2__
 @import _Builtin_intrinsics.intel.avx2;
 #endif