use /imsvc with clang-cl where we use -isystem

This should disable warnings from third-party headers
on Clang/Win builds.  So far we've just gotten lucky.

Change-Id: Ieaf459e200925d46d8c65ba8d489db111705b125
Reviewed-on: https://skia-review.googlesource.com/c/191283
Commit-Queue: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
1 file changed