Do not print include_next/pragma once warnings when input is a header.

r276653 suppressed the pragma once warning when generating a PCH file.
This patch extends that to any main file for which clang is told (with
the -x option) that it's a header file. It will also suppress the
warning "#include_next in primary source file".

Differential Revision: http://reviews.llvm.org/D25989

llvm-svn: 285295