clang-cl: Various changes to /Zc: handling.

* /Zc:trigraphs and /Zc:trigraphs- are now honored
* /Zc:strictStrings is now honored
* /Zc:auto is now honored/ignored (clang does the Right Thing for this already)

Also add a dedicated test for the various /Zc: flags.
clang-cl doesn't always agree with cl.exe on the default values for /Zc flags.
For example, I think clang always behaves as if /Zc:inline is passed, and
warns if the user explicitly passes /Zc:inline-

Fixes PR21974.

llvm-svn: 224791