Add an option for clang-cl to emit clang style diagnostics

clang-cl's diagnostics messages are different to clang++ or gcc. That is
not very convenient for the editor support like Emacs compilation mode.

This CL adds a configuration flag, use_clang_diagnostics_format, to use
clang/gcc style diagnostics.

Change-Id: I92599c6025d36af7eff74454d58e2f74e516c627
Reviewed-on: https://chromium-review.googlesource.com/1061034
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584364}

build/config/win/BUILD.gn

@@ -37,6 +37,16 @@ declare_args() {
   #   "server" - Windows Server Applications
   #   "desktop" - Windows Desktop Applications
   target_winuwp_family = "app"
+
+  # Set this to use clang style diagnostics format. Available only on clang.
+  # Tune this on for the editor integrations like Emacs compilation mode that
+  # match clang style error messages.
+  # E.g.:
+  #  Without this, clang emits a diagnostic message like this:
+  #    foo/bar.cc(12,34): error: something went wrong
+  #  and with this switch, clang emits it like this:
+  #    foo/bar.cc:12:34: error: something went wrong
+  use_clang_diagnostics_format = false
 }
 
 # This is included by reference in the //build/config/compiler config that
@@ -93,6 +103,15 @@ config("compiler") {
       ]
     }
 
+    if (is_clang && use_clang_diagnostics_format) {
+      cflags += [
+        "-Xclang",
+        "-fdiagnostics-format",
+        "-Xclang",
+        "clang",
+      ]
+    }
+
     # Clang runtime libraries, such as the sanitizer runtimes, live here.
     lib_dirs = [ "$clang_base_path/lib/clang/$clang_version/lib/windows" ]
   }