use ghash linking on Windows

Should be the same symbol level, but linked faster.

see
https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/c7Ecb7NF5Ik

and
https://bugs.chromium.org/p/chromium/issues/detail?id=904324

and
http://blog.llvm.org/2018/01/improving-link-time-on-windows-with.html

Change-Id: I67d1b14488f5238a1e658d3e05d94809e69957a1
Reviewed-on: https://skia-review.googlesource.com/c/180100
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>

gn/BUILD.gn

@@ -428,7 +428,8 @@ config("debug_symbols") {
   } else if (is_win) {
     cflags = [ "/Z7" ]
     if (is_clang) {
-      ldflags = [ "/DEBUG" ]
+      cflags += [ "-mllvm", "-emit-codeview-ghash-section" ]
+      ldflags = [ "/DEBUG:GHASH" ]
     } else {
       ldflags = [ "/DEBUG:FASTLINK" ]
     }