preprocessor: Reinitialize frontend parser after loading a PCH [PR112319]

preprocessor: Reinitialize frontend parser after loading a PCH [PR112319]

Since r14-2893, the frontend parser object needs to exist when running in
preprocess-only mode, because pragma_lex() is now called in that mode and
needs to make use of it. This is handled by calling c_init_preprocess() at
startup. If -fpch-preprocess is in effect (commonly, because of
-save-temps), a PCH file may be loaded during preprocessing, in which
case the parser will be destroyed, causing the issue noted in the
PR. Resolve it by reinitializing the frontend parser after loading the PCH.

gcc/c-family/ChangeLog:

	PR pch/112319
	* c-ppoutput.cc (cb_read_pch): Reinitialize the frontend parser
	after loading a PCH.

gcc/testsuite/ChangeLog:

	PR pch/112319
	* g++.dg/pch/pr112319.C: New test.
	* g++.dg/pch/pr112319.Hs: New test.
	* gcc.dg/pch/pr112319.c: New test.
	* gcc.dg/pch/pr112319.hs: New test.