c++: modules & -fpreprocessed [PR 99072]

When we read preprocessed source, we deal with a couple of special
location lines at the start of the file.  These provide information
about the original filename of the source and the current directory,
so we can process the source in the same manner.  When updating that
code, I had a somewhat philosophical question: Should the line table
contain evidence of the filename the user provided to the compiler?  I
figured to leave it there, as it did no harm.  But this defect shows
an issue.  It's in the line table and our (non optimizing) line table
serializer emits that filename.  Which means if one re-preprocesses
the original source to a differently-named intermediate file, the
resultant CMI is different.  Boo.  That's a difference that doesn't
matter, except the CRC matching then fails.  We should elide the
filename, so that one can preprocess to mktemp intermediate filenames
for whatever reason.

This patch takes the approach of expunging it from the line table --
so the line table will end up with exactly the same form.  That seems
a better bet than trying to fix up mismatching line tables in CMI
emission.

	PR c++/99072
	libcpp/
	* init.c (read_original_filename): Expunge all evidence of the
	original filename.
	gcc/testsuite/
	* g++.dg/modules/pr99072.H: New.

gcc/testsuite/g++.dg/modules/pr99072.H

@@ -0,0 +1,10 @@
+# 0 "REALNAME"
+# 0 "<built-in>"
+# 0 "<command-line>"
+# 0 "<command-line>"
+# 1 "/usr/include/stdc-predef.h" 1 3 4
+# 0 "<command-line>" 2
+# 1 "REALNAME"
+
+// { dg-additional-options {-fmodule-header -fpreprocessed -fdump-lang-module-lineno} }
+// { dg-final { scan-lang-dump { 4 source file names\n Source file\[0\]=REALNAME\n Source file\[1\]=<built-in>\n Source file\[2\]=<command-line>\n Source file\[3\]=/usr/include/stdc-predef.h\n} module } }

libcpp/init.c

@@ -752,6 +752,23 @@ read_original_filename (cpp_reader *pfile)
       if (_cpp_handle_directive (pfile, token->flags & PREV_WHITE))
 	{
 	  read_original_directory (pfile);
+
+	  auto *penult = &linemap_check_ordinary
+	    (LINEMAPS_LAST_MAP (pfile->line_table, false))[-1];
+	  if (penult[1].reason == LC_RENAME_VERBATIM)
+	    {
+	      /* Expunge any evidence of the original linemap.  */
+	      pfile->line_table->highest_location
+		= pfile->line_table->highest_line
+		= penult[0].start_location;
+
+	      penult[1].start_location = penult[0].start_location;
+	      penult[1].reason = penult[0].reason;
+	      penult[0] = penult[1];
+	      pfile->line_table->info_ordinary.used--;
+	      pfile->line_table->info_ordinary.cache = 0;
+	    }
+
 	  return true;
 	}
     }