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.

libcpp/include/cpplib.h

@@ -979,7 +979,8 @@ extern class mkdeps *cpp_get_deps (cpp_reader *) ATTRIBUTE_PURE;
    input file, except for preprocessed input.  This will generate at
    least one file change callback, and possibly a line change callback
    too.  If there was an error opening the file, it returns NULL.  */
-extern const char *cpp_read_main_file (cpp_reader *, const char *);
+extern const char *cpp_read_main_file (cpp_reader *, const char *,
+				       bool injecting = false);
 
 /* Set up built-ins with special behavior.  Use cpp_init_builtins()
    instead unless your know what you are doing.  */