[PATCH] Teach Clang how to use response files when calling other tools

[Sean Silva]

This is looking a lot better.

In particular, I feel like the testing needs a bit more work; I'm not seeing the connection between the added tests and the functional changes in this patch.

I'd like espindola to give a second opinion on the code, and he can probably suggest improvements to the testing.

================
Comment at: include/clang/Driver/Job.h:103
@@ +102,3 @@
+  bool useResponseFile() const {
+    return ResponseFile != nullptr && NeedsResponseFile;
+  }
----------------
Is there ever any situation where NeedsResponseFile == true but ResponseFile != nullptr? In general, what is the meaning of the 4 different possibilities for

(ResponseFile {=,!}= nullptr) x (NeedsResponseFile {=,!}= true)

http://reviews.llvm.org/D4897