[clang-cl, PCH] Support for /Yc and /Yu without filename and #pragma hdrstop

With clang-cl, when the user specifies /Yc or /Yu without a filename
the compiler uses a #pragma hdrstop in the main source file to
determine the end of the PCH. If a header is specified with /Yc or
/Yu #pragma hdrstop has no effect.

The optional #pragma hdrstop filename argument is not yet supported.

Differential Revision: https://reviews.llvm.org/D51391

llvm-svn: 341963