Set -fmsc-version appropriately for clang on VS2015

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 48ab1cb85298e6c1d201fc7394e5cb33335a1c06..f7797958044ce13f30fbba5f579e81fd282ed097 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -108,12 +108,17 @@ config("compiler") {
     # Keep this in sync with the similar block in build/common.gypi
     if (is_clang) {
       cflags += [
-        "-fmsc-version=1800",
-
         # Many files use intrinsics without including this header.
         # TODO(hans): Fix those files, or move this to sub-GYPs.
         "/FIIntrin.h",
       ]
+
+      if (visual_studio_version == "2013") {

[hans, 2015/06/10 18:05:38]

This buildbot seems sad because of this:
http://build.chromium.org/p/chromium.fyi/builders/CrWinClang64%28dbg%29/build...

+        cflags += [ "-fmsc-version=1800" ]
+      } else if (visual_studio_version == "2015") {
+        cflags += [ "-fmsc-version=1900" ]
+      }
+
       if (current_cpu == "x86") {
         cflags += [
           "/fallback",