Allow /showIncludes with /P

r213589 was checked in as a solution to
https://bugs.llvm.org/show_bug.cgi?id=20336.

However, it is possible to use /EP with /P
to suppress #line directives AND output to
a file. There is no reason in that case to
suppress /showIncludes.

This was reported here:
https://bugs.llvm.org/show_bug.cgi?id=34997

Differential Revision: https://reviews.llvm.org/D39104

llvm-svn: 316225