preprocessor: Better line info for <builtin> & <command-line>

With C++ module header units it becomes important to distinguish
between macros defined in forced headers (& commandline & builtins)
from those defined in the header file being processed.  We weren't
making that easy because we treated the builtins and command-line
locations somewhat file-like, with incrementing line numbers, and
showing them as included from line 1 of the main file.  This patch does
3 things:

0) extend the idiom that 'line 0' of a file means 'the file as a whole'

1) builtins and command-line macros are shown as-if included from line zero.

2) when emitting preprocessed output we keep resetting the line number
so that re-reading that preprocessed output will get the same set of
locations for the command line etc.

For instance the new c-c++-common/cpp/line-2.c test, now emits

In file included from <command-line>:
./line-2.h:4:2: error: #error wrong
    4 | #error wrong
      |  ^~~~~
line-2.c:3:11: error: macro "bill" passed 1 arguments, but takes just 0
    3 | int bill(1);
      |           ^
In file included from <command-line>:
./line-2.h:3: note: macro "bill" defined here
    3 | #define bill() 2
      |

Before it told you about including from <command-line>:31.

the preprocessed output looks like:
...

(There's a new optimization in do_line_marker to stop each of these
line markers causing a new line map.  We can simply rewind the
location, and keep using the same line map.)

	libcpp/
	* directives.c (do_linemarker): Optimize rewinding to line zero.
	* files.c (_cpp_stack_file): Start on line zero when about to inject
	headers.
	(cpp_push_include, cpp_push_default_include): Use highest_line as
	the location.
	* include/cpplib.h (cpp_read_main_file): Add injecting parm.
	* init.c (cpp_read_main_file): Likewise, inform _cpp_stack_file.
	* internal.h (enum include_type): Add IT_MAIN_INJECT.
	gcc/c-family/
	* c-opts.c (c_common_post_options): Add 'injecting' arg to
	cpp_read_main_file.
	(c_finish_options): Add linemap_line_start calls for builtin and cmd
	maps.  Force token position to line_table's highest line.
	* c-ppoutput.c (print_line_1): Refactor, print line zero.
	(cb_define): Always increment source line.
	gcc/testsuite/
	* c-c++-common/cpp/line-2.c: New.
	* c-c++-common/cpp/line-2.h: New.
	* c-c++-common/cpp/line-3.c: New.
	* c-c++-common/cpp/line-4.c: New.
	* c-c++-common/cpp/line-4.h: New.

2 files changed, 14 insertions, 12 deletions

diff --git a/gcc/c-family/c-opts.c b/gcc/c-family/c-opts.c
index 9b6300f..ec3de86 100644
--- a/gcc/c-family/c-opts.c
+++ b/gcc/c-family/c-opts.c
@@ -1110,7 +1110,11 @@ c_common_post_options (const char **pfilename)
   input_location = UNKNOWN_LOCATION;
 
   *pfilename = this_input_filename
-    = cpp_read_main_file (parse_in, in_fnames[0]);
+    = cpp_read_main_file (parse_in, in_fnames[0],
+			  /* We'll inject preamble pieces if this is
+			     not preprocessed.  */
+			  !cpp_opts->preprocessed);
+
   /* Don't do any compilation or preprocessing if there is no input file.  */
   if (this_input_filename == NULL)
     {
@@ -1429,6 +1433,7 @@ c_finish_options (void)
 	= linemap_check_ordinary (linemap_add (line_table, LC_RENAME, 0,
 					       _("<built-in>"), 0));
       cb_file_change (parse_in, bltin_map);
+      linemap_line_start (line_table, 0, 1);
 
       /* Make sure all of the builtins about to be declared have
 	 BUILTINS_LOCATION has their location_t.  */
@@ -1452,9 +1457,10 @@ c_finish_options (void)
 	= linemap_check_ordinary (linemap_add (line_table, LC_RENAME, 0,
 					       _("<command-line>"), 0));
       cb_file_change (parse_in, cmd_map);
+      linemap_line_start (line_table, 0, 1);
 
       /* All command line defines must have the same location.  */
-      cpp_force_token_locations (parse_in, cmd_map->start_location);
+      cpp_force_token_locations (parse_in, line_table->highest_line);
       for (size_t i = 0; i < deferred_count; i++)
 	{
 	  struct deferred_opt *opt = &deferred_opts[i];
diff --git a/gcc/c-family/c-ppoutput.c b/gcc/c-family/c-ppoutput.c
index 1e2b32b..44c6f30 100644
--- a/gcc/c-family/c-ppoutput.c
+++ b/gcc/c-family/c-ppoutput.c
@@ -560,26 +560,23 @@ print_line_1 (location_t src_loc, const char *special_flags, FILE *stream)
   if (src_loc != UNKNOWN_LOCATION && !flag_no_line_commands)
     {
       const char *file_path = LOCATION_FILE (src_loc);
-      int sysp;
       size_t to_file_len = strlen (file_path);
       unsigned char *to_file_quoted =
          (unsigned char *) alloca (to_file_len * 4 + 1);
-      unsigned char *p;
 
       print.src_line = LOCATION_LINE (src_loc);
       print.src_file = file_path;
 
       /* cpp_quote_string does not nul-terminate, so we have to do it
 	 ourselves.  */
-      p = cpp_quote_string (to_file_quoted,
-			    (const unsigned char *) file_path,
-			    to_file_len);
+      unsigned char *p = cpp_quote_string (to_file_quoted,
+					   (const unsigned char *) file_path,
+					   to_file_len);
       *p = '\0';
       fprintf (stream, "# %u \"%s\"%s",
-	       print.src_line == 0 ? 1 : print.src_line,
-	       to_file_quoted, special_flags);
+	       print.src_line, to_file_quoted, special_flags);
 
-      sysp = in_system_header_at (src_loc);
+      int sysp = in_system_header_at (src_loc);
       if (sysp == 2)
 	fputs (" 3 4", stream);
       else if (sysp == 1)
@@ -677,8 +674,7 @@ cb_define (cpp_reader *pfile, location_t line, cpp_hashnode *node)
   linemap_resolve_location (line_table, line,
 			    LRK_MACRO_DEFINITION_LOCATION,
 			    &map);
-  if (LINEMAP_LINE (map) != 0)
-    print.src_line++;
+  print.src_line++;
 }
 
 static void