[gn] Use ghash if using clang & LLD together to make PDBs

[rnk]

Summary

I noticed my links were a bit slower on Windows than usual.

[dblaikie]

Just a quick note - PHabricator has a bug where it doesn't send email when status changes are made without text. @thakis please include some text when approving patches to ensure that approval is reflected on the mailing list as well.

llvm/utils/gn/build/BUILD.gn

if (host_os != "win") {
     ]
   } else {
     if (is_debug) {
       cflags += [
         "/Zi",
         "/FS",
       ]
       ldflags += [ "/DEBUG" ]
+
+      # Speed up links with ghash on windows.
+      if (use_lld && is_clang) {
+        cflags += [ "-gcodeview-ghash" ]
+        ldflags += [ "/DEBUG:GHASH" ]
+      }
     }
     if (is_optimized) {
       cflags += [
         "/O2",
         "/Gw",
         "/Zc:inline",
       ]
       ldflags += [