Tell clang/win to emulate MSVC 2015

By default it emulates your installed cl.exe, but the bots don't have
one.  I think the fallback is 2013, which causes all sorts of pre-C++11
problems.

CQ_INCLUDE_TRYBOTS=skia.primary:Build-Win-Clang-x86_64-Release

Change-Id: I2556abe68825e58762b4172d067ba6826de5c133
Reviewed-on: https://skia-review.googlesource.com/29021
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
diff --git a/gn/BUILD.gn b/gn/BUILD.gn
index e1cca92..7be1617 100644
--- a/gn/BUILD.gn
+++ b/gn/BUILD.gn
@@ -44,6 +44,9 @@
       "/WX",  # Treat warnings as errors.
       "/utf-8",  # Set Source and Executable character sets to UTF-8.
     ]
+    if (is_clang) {
+      cflags += [ "-fms-compatibility-version=19" ]  # 2015
+    }
     defines += [
       "_CRT_SECURE_NO_WARNINGS",  # Disables warnings about sscanf().
       "_HAS_EXCEPTIONS=0",  # Disables exceptions in MSVC STL.