Fix test by using -target instead of cc1 arguments (c-index-test goes through the driver)

[filcab]

Summary

If LLVM_DEFAULT_TARGET_TRIPLE is a non-darwin triple, the file might
fail. This might allow us to take out the FIXME and REQUIRES lines, but I'd
rather let the bots double-check that the test is ok first.

[filcab]

Ping!

[akyrtzi]

FYI, I had added -target in this test in r296263 but it was still failing for some reason in some bots, that's why I then switched to triple via r296265.
I'm not sure if it's going work now or not for all bots, but it does, that is definitely better.

test/Index/pch-from-libclang.c

 // Check that clang can use a PCH created from libclang.
 
 // FIXME: Non-darwin bots fail. Would need investigation using -module-file-info to see what is the difference in modules generated from libclang vs the compiler invocation, in those systems.
 // REQUIRES: system-darwin
 
 // RUN: %clang_cc1 -fsyntax-only %s -verify
-// RUN: c-index-test -write-pch %t.h.pch %s -fmodules -fmodules-cache-path=%t.mcp -Xclang -triple -Xclang x86_64-apple-darwin
-// RUN: %clang -fsyntax-only -include %t.h %s -Xclang -verify -fmodules -fmodules-cache-path=%t.mcp -Xclang -detailed-preprocessing-record -Xclang -triple -Xclang x86_64-apple-darwin -Xclang -fallow-pch-with-compiler-errors
-// RUN: %clang -x c-header %s -o %t.clang.h.pch -fmodules -fmodules-cache-path=%t.mcp -Xclang -detailed-preprocessing-record -Xclang -triple -Xclang x86_64-apple-darwin -Xclang -fallow-pch-with-compiler-errors -Xclang -verify
-// RUN: c-index-test -test-load-source local %s -include %t.clang.h -fmodules -fmodules-cache-path=%t.mcp -Xclang -triple -Xclang x86_64-apple-darwin | FileCheck %s
+// RUN: c-index-test -write-pch %t.h.pch %s -fmodules -fmodules-cache-path=%t.mcp -target x86_64-apple-darwin
+// RUN: %clang -fsyntax-only -include %t.h %s -Xclang -verify -fmodules -fmodules-cache-path=%t.mcp -Xclang -detailed-preprocessing-record -target x86_64-apple-darwin -Xclang -fallow-pch-with-compiler-errors
+// RUN: %clang -x c-header %s -o %t.clang.h.pch -fmodules -fmodules-cache-path=%t.mcp -Xclang -detailed-preprocessing-record -target x86_64-apple-darwin -Xclang -fallow-pch-with-compiler-errors -Xclang -verify
+// RUN: c-index-test -test-load-source local %s -include %t.clang.h -fmodules -fmodules-cache-path=%t.mcp -target x86_64-apple-darwin | FileCheck %s
 
 #ifndef HEADER
 #define HEADER
 
 void some_function(undeclared_type p); // expected-error{{unknown type name}}
 
 struct S { int x; };