Re-enable microsoft pragma warning

We currently suppress clang's unknown-pragma warning about microsoft's #pragma optimize. There was a lot of work done in the past (http://crbug.com/912662) to remove our uses of these pragmas, which ended with all warnings removed except those in MIDL-generated code.

Since then, LLVM has been updated to only warn on non-empty uses of #pragma optimize (https://github.com/llvm/llvm-project/commit/186bea3750d6b349de0e71044d95e2a42e087b4c). Since we don't have any, we can re-enable the warning.

Bug: 912662
Change-Id: Ia28fe228a324f13e86f0b3703c119968f5aad43a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5688955
Reviewed-by: Alan Zhao <ayzhao@google.com>
Commit-Queue: Devon Loehr <dloehr@google.com>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1326112}