clang-cl: Add a `/showIncludes:user` flag.

clang-cl: Add a `/showIncludes:user` flag.

This flag is like /showIncludes, but it only includes user headers and
omits system headers (similar to MD and MMD). The motivation is that
projects that already track system includes though other means can use
this flag to get consistent behavior on Windows and non-Windows, and it
saves tools that output /showIncludes output (e.g. ninja) some work.

implementation-wise, this makes `HeaderIncludesCallback` honor the
existing `IncludeSystemHeaders` bit, and changes the three clients of
`HeaderIncludesCallback` (`/showIncludes`, `-H`, `CC_PRINT_HEADERS=1`)
to pass `-sys-header-deps` to set that bit -- except for
`/showIncludes:user`, which doesn't pass it.

Differential Revision: https://reviews.llvm.org/D75093