libcpp: use better locations for _Pragma tokens (preprocessor/69126)

Our code for handling the "_Pragma" builtin macro is implemented in
libcpp/directives.c:destringize_and_run.  

It handles _Pragma by creating a one-line buffer containing the _Pragma
content, then sending it to do_pragma, which tokenizes it and handles
the input as if it were spelled as #pragma.

Unfortunately the tokens it generates have bogus location values; the
values are in the current highest ordinary map at the time of expansion,
and this determines the effective location of the synthesized #pragma.

Hence for PR preprocessor/69126:

VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV
#pragma GCC diagnostic push
#define MACRO \
    _Pragma("GCC diagnostic ignored \"-Wunused-variable\"") \
    int x;

int g()
{
    MACRO;
    return 0;
}
#pragma GCC diagnostic pop
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

although -save-temps looks sane:

VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV
#pragma GCC diagnostic push

int g()
{
# 19 "../../src/gcc/testsuite/c-c++-common/pr69126.c"
#pragma GCC diagnostic ignored "-Wunused-variable"
# 19 "../../src/gcc/testsuite/c-c++-common/pr69126.c"
    int x;;
    return 0;
}
#pragma GCC diagnostic pop
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

the "ignored" directive is treated as if it occured in this
bogus location:

(gdb) call inform (226930, "pre, the location of the ignore")
/tmp/test.cc:17:24: note: pre, the location of the ignore
     MACRO;
                        ^
which is effectively *after* the
    int x;;
line.

I believe this is a long-standing bug, but the cleanup of the
C++ FE's impl of -Wunused-variable in r226234 (to avoid using
%q+D) has exposed this as a regression (since the -Wunused-variable
warning is now emitted for this location:
    int x;
         ^
rather than at input_location:
     return 0;
             ^
and hence isn't suppressed by the _Pragma.

The function destringize_and_run is full of comments like
  /* Ugh; an awful kludge.  We are really not set up to be lexing
and:
  /* ??? Antique Disgusting Hack.  What does this do?  */
and similar "???" lines.

I believe it predates macro maps (I believe they were added in
r180081 on 2011-10-15); as far as I can tell, the code is set
up for *line* maps, i.e. it may even predate column information
(it gets the line right, but not the column).

To minimize the change to destringize_and_run at this stage, the
following patch papers over the problem by fixing up the locations
for the tokens synthesized for _Pragma, setting them all to be at the
expansion point of the _Pragma directive, rather than some columns
after it.

This fixes the location of the synthesized #pragma and hence
fixes the regression seen in the PR.  The -save-temps output (which
was already correct) is unaffected.

Successfully bootstrapped&regrtested on x86_64-pc-linux-gnu;
adds 3 PASS results to g++.sum and 1 to gcc.sum
(although the regression currently only affects C++, I put the new
test case into c-c++-common for good measure).

OK for trunk in stage 3?

gcc/testsuite/ChangeLog:
	PR preprocessor/69126
	* c-c++-common/pr69126.c: New test case.

libcpp/ChangeLog:
	PR preprocessor/69126
	* directives.c (destringize_and_run): Add expansion_loc param; use
	it when handling unexpanded pragmas to fixup the locations of the
	synthesized tokens.
	(_cpp_do__Pragma): Add expansion_loc param and use it when calling
	destringize_and_run.
	* internal.h (_cpp_do__Pragma): Add expansion_loc param.
	* macro.c (builtin_macro): Pass expansion location of _Pragma to
	_cpp_do__Pragma.
---
 gcc/testsuite/c-c++-common/pr69126.c | 22 ++++++++++++++++++++++
 libcpp/directives.c                  | 13 ++++++++++---
 libcpp/internal.h                    |  2 +-
 libcpp/macro.c                       |  2 +-
 4 files changed, 34 insertions(+), 5 deletions(-)
 create mode 100644 gcc/testsuite/c-c++-common/pr69126.c

libcpp: use better locations for _Pragma tokens (preprocessor/69126)

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