Add ability to turn off -fpch-instantiate-templates in clang-cl
A lot of our code building with clang-cl.exe using Clang 11 was failing with
the following 2 type of errors:
- explicit specialization of 'foo' after instantiation
- no matching function for call to 'bar'
Note that we also use -fdelayed-template-parsing in our builds.
I tried pretty hard to get a small repro for these failures, but couldn't. So
there is some subtle edge case in the -fpch-instantiate-templates feature
introduced by this change: https://reviews.llvm.org/D69585
When I tried turning this off using -fno-pch-instantiate-templates, builds
would silently fail with the same error without any indication that
-fno-pch-instantiate-templates was being ignored by the compiler. Then I
realized this "no" option wasn't actually working when I ran Clang under a
Differential revision: https://reviews.llvm.org/D88680