Use -fdiagnostics-absolute-paths in Win-Clang builds

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index aa22d90bfdec0c78ae06ef1d3b7b62a41e75751e..83132ae8edeef328b862bc7cfae4955df4ebaccb 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -372,6 +372,12 @@ config("compiler") {
     cflags += [ "-fcolor-diagnostics" ]
   }
 
+  # Windows devs want absolute paths in diagnostics (crbug.com/636109).

[Nico, 2016/08/26 17:06:24]

Maybe you can expand this a bit based on discussion ("why only windows?)"

[hans, 2016/08/26 20:50:06]

Done.

+  # TODO(hans) Remove llvm_force_head_revision check in next clang roll.
+  if (is_clang && is_win && llvm_force_head_revision) {

[Nico, 2016/08/26 16:13:01]

Should we do this on all platforms? Seems a bit arbitrary to do this on Windows
but not elsewhere.

[hans, 2016/08/26 16:21:43]

I agree it might seem a bit odd to do it on only one platform, but I think the
desire for this is very Windows-specific.

Personally I prefer relative paths, and I think this is very natural to most
Linux and Mac folks too.

I guess what it boils down to is that there's no precedence for absolute paths
in diagnostics on Unix (GCC doesn't do it), but there is on Windows and those
developers seem to expect it.

[scottmg, 2016/08/26 16:58:34]

Yeah, I would agree with keeping it Windows-only. Even though it's a bit odd, 
with /FC in gyp/gn it's been different on both platforms for a long time.

(We have /FC on when running like this, right? Does this also need to be turned
on explicitly still for clang-cl?)

+    cflags += [ "-fdiagnostics-absolute-paths" ]
+  }
+
   # Makes builds independent of absolute file path.
   # clang-cl (used if is_win) doesn't expose this flag.
   # Currently disabled for nacl since its toolchain lacks this flag (too old).