preprocessor: Create the parser before handling command-line includes [PR115312]

Since r14-2893, we create a parser object in preprocess-only mode for the
purpose of parsing #pragma while preprocessing. The parser object was
formerly created after calling c_finish_options(), which leads to problems
on platforms that don't use stdc-predef.h (such as MinGW, as reported in
the PR). On such platforms, the call to c_finish_options() will process
the first command-line-specified include file. If that includes a PCH, then
c-ppoutput.cc will encounter a state it did not anticipate. Fix it by
creating the parser prior to calling c_finish_options().

gcc/c-family/ChangeLog:

	PR pch/115312
	* c-opts.cc (c_common_init): Call c_init_preprocess() before
	c_finish_options() so that a parser is available to process any
	includes specified on the command line.

gcc/testsuite/ChangeLog:

	PR pch/115312
	* g++.dg/pch/pr115312.C: New test.
	* g++.dg/pch/pr115312.Hs: New test.