c-family: Fix ICE with large column number after restoring a PCH [PR105608]

Users are allowed to define macros prior to restoring a precompiled header
file, as long as those macros are not defined (or are defined identically)
in the PCH.  However, the PCH restoration process destroys all the macro
definitions, so libcpp has to record them before restoring the PCH and then
redefine them afterward.

This process does not currently assign great locations to the macros after
redefining them. Some work is needed to also remember the original locations
and get the line_maps instance in the right state (since, like all other
data structures, the line_maps instance is also reset after restoring a PCH).
The new testcase line-map-3.C contains XFAILed examples where the locations
are wrong.

This patch addresses a more pressing issue, which is that we ICE in some
cases since GCC 11, hitting an assert in line-maps.cc. It happens if the
first line encountered after the PCH restore requires an LC_RENAME map, such
as will happen if the line is sufficiently long.  This is much easier to
fix, since we just need to call linemap_line_start before asking libcpp to
redefine the stored macros, instead of afterward, to avoid the unexpected
need for an LC_RENAME before an LC_ENTER has been seen.

gcc/c-family/ChangeLog:

	PR preprocessor/105608
	* c-pch.cc (c_common_read_pch): Start a new line map before asking
	libcpp to restore macros defined prior to reading the PCH, instead
	of afterward.

gcc/testsuite/ChangeLog:

	PR preprocessor/105608
	* g++.dg/pch/line-map-1.C: New test.
	* g++.dg/pch/line-map-1.Hs: New test.
	* g++.dg/pch/line-map-2.C: New test.
	* g++.dg/pch/line-map-2.Hs: New test.
	* g++.dg/pch/line-map-3.C: New test.
	* g++.dg/pch/line-map-3.Hs: New test.

gcc/c-family/c-pch.cc

@@ -342,6 +342,8 @@ c_common_read_pch (cpp_reader *pfile, const char *name,
   gt_pch_restore (f);
   cpp_set_line_map (pfile, line_table);
   rebuild_location_adhoc_htab (line_table);
+  line_table->trace_includes = saved_trace_includes;
+  linemap_add (line_table, LC_ENTER, 0, saved_loc.file, saved_loc.line);
 
   timevar_push (TV_PCH_CPP_RESTORE);
   if (cpp_read_state (pfile, name, f, smd) != 0)
@@ -355,9 +357,6 @@ c_common_read_pch (cpp_reader *pfile, const char *name,
 
   fclose (f);
 
-  line_table->trace_includes = saved_trace_includes;
-  linemap_add (line_table, LC_ENTER, 0, saved_loc.file, saved_loc.line);
-
   /* Give the front end a chance to take action after a PCH file has
      been loaded.  */
   if (lang_post_pch_load)

gcc/testsuite/g++.dg/pch/line-map-1.C

@@ -0,0 +1,4 @@
+/* PR preprocessor/105608 */
+/* { dg-do compile } */
+#define MACRO_ON_A_LONG_LINE "this line is long enough that it forces the line table to create an LC_RENAME map, which formerly triggered an ICE after PCH restore"
+#include "line-map-1.H"

gcc/testsuite/g++.dg/pch/line-map-1.Hs

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

gcc/testsuite/g++.dg/pch/line-map-2.C

@@ -0,0 +1,6 @@
+/* PR preprocessor/105608 */
+/* { dg-do compile } */
+/* { dg-additional-options "-save-temps" } */
+#define MACRO_ON_A_LONG_LINE "this line is long enough that it forces the line table to create an LC_RENAME map, which formerly triggered an ICE after PCH restore"
+#include "line-map-2.H"
+#error "suppress PCH assembly comparison, which does not work with -save-temps" /* { dg-error "." } */

gcc/testsuite/g++.dg/pch/line-map-2.Hs

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

gcc/testsuite/g++.dg/pch/line-map-3.C

@@ -0,0 +1,23 @@
+#define UNUSED_MACRO /* { dg-error "UNUSED_MACRO" "" { xfail *-*-* } } */
+#include "line-map-3.H" /* { dg-bogus "-:UNUSED_MACRO" "" { xfail *-*-* } } */
+
+/* { dg-do compile } */
+/* { dg-additional-options "-Werror=unused-macros" } */
+
+/* PR preprocessor/105608 */
+/* This test case is currently xfailed and requires work in libcpp/pch.cc to
+   resolve.  Currently, the macro location is incorrectly assigned to line 2
+   of the header file when read via PCH, because libcpp doesn't try to
+   assign locations relative to the newly loaded line map after restoring
+   the PCH.  */
+
+/* In PCH mode we also complain incorrectly about the command line macro -Dwith_PCH
+   added by dejagnu; that warning would get suppressed if the macro location were
+   correctly restored by libcpp to reflect that it was a command line macro.  */
+/* { dg-bogus "-:with_PCH" "" { xfail *-*-* } 2 } */
+
+/* The reason we used -Werror was to prevent pch.exp from rerunning without PCH;
+   in that case we would get unnecessary XPASS outputs since the test does work
+   fine without PCH.  Once the bug is fixed, remove the -Werror and switch to
+   dg-warning.  */
+/* { dg-regexp {[^[:space:]]*: some warnings being treated as errors} } */

gcc/testsuite/g++.dg/pch/line-map-3.Hs

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