c-family: Let libcpp know when the compilation is for a PCH [PR9471]

libcpp will generate diagnostics when it encounters things in the main file
that only belong in a header file, such as `#pragma once' or `#pragma GCC
system_header'. But sometimes the main file is a header file that is just
being compiled separately, e.g. to produce a C++ module or a PCH, in which
case such diagnostics should be suppressed. libcpp already has an interface
to request that, so make use of it in the C frontends to prevent libcpp from
issuing unwanted diagnostics when compiling a PCH.

gcc/c-family/ChangeLog:

	PR pch/9471
	PR pch/47857
	* c-opts.cc (c_common_post_options): Set cpp_opts->main_search
	so libcpp knows it is compiling a header file separately.

gcc/testsuite/ChangeLog:

	PR pch/9471
	PR pch/47857
	* g++.dg/pch/main-file-warnings.C: New test.
	* g++.dg/pch/main-file-warnings.Hs: New test.
	* gcc.dg/pch/main-file-warnings.c: New test.
	* gcc.dg/pch/main-file-warnings.hs: New test.