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.

gcc/c-family/c-ppoutput.cc

@@ -862,4 +862,9 @@ cb_read_pch (cpp_reader *pfile, const char *name,
 
   fprintf (print.outf, "#pragma GCC pch_preprocess \"%s\"\n", name);
   print.src_line++;
+
+  /* The process of reading the PCH has destroyed the frontend parser,
+     so ask the frontend to reinitialize it, in case we need it to
+     process any #pragma directives encountered while preprocessing.  */
+  c_init_preprocess ();
 }

gcc/testsuite/g++.dg/pch/pr112319.C

@@ -0,0 +1,5 @@
+/* { dg-additional-options "-Wpragmas -save-temps" } */
+#include "pr112319.H"
+#pragma GCC diagnostic error "-Wpragmas"
+#pragma GCC diagnostic ignored "oops" /* { dg-error "oops" } */
+/* { dg-regexp {[^[:space:]]*: some warnings being treated as errors} } */

gcc/testsuite/g++.dg/pch/pr112319.Hs

@@ -0,0 +1 @@
+/* This space intentionally left blank.  */

gcc/testsuite/gcc.dg/pch/pr112319.c

@@ -0,0 +1,5 @@
+/* { dg-additional-options "-Wpragmas -save-temps" } */
+#include "pr112319.h"
+#pragma GCC diagnostic error "-Wpragmas"
+#pragma GCC diagnostic ignored "oops" /* { dg-error "oops" } */
+/* { dg-regexp {[^[:space:]]*: some warnings being treated as errors} } */

gcc/testsuite/gcc.dg/pch/pr112319.hs

@@ -0,0 +1 @@
+/* This space intentionally left blank.  */