Apply msvs_use_absolute_paths gn argument to clang.

Make clang build use msvs_use_absolute_paths gn argument to get same
behavior in msvc & clang builds.

https://chromium-review.googlesource.com/c/558871/#message-7a8946793945959784b225c192588fa4bb5f26b1

Follow-up: https://chromium-review.googlesource.com/c/558871/
Bug: 
Change-Id: I9c35d83b0f163356aac0fd2c94f4c56c02a794a5
Reviewed-on: https://chromium-review.googlesource.com/585467Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491149}

build/config/compiler/BUILD.gn

@@ -436,7 +436,7 @@ config("compiler") {
   # Print absolute paths in diagnostics. There is no precedent for doing this
   # on Linux/Mac (GCC doesn't support it), but MSVC does this with /FC and
   # Windows developers rely on it (crbug.com/636109) so only do this on Windows.
-  if (is_clang && is_win) {
+  if (msvc_use_absolute_paths && is_clang && is_win) {
     cflags += [ "-fdiagnostics-absolute-paths" ]
   }