Strip "<built-in>" loc from displayed expansion context

Now that diagnostics for tokens coming from macro expansions point to
the spelling location of the relevant token (and then displays the
context of the expansion), some ugly (not so seldom) corner cases can
happen.

When the relevant token is a built-in token (which means the location
of that token is BUILTINS_LOCATION) the location prefix displayed to
the user in the diagnostic line is the "<built-in>:0:0" string.  For
instance:

    <built-in>:0:0: warning: conversion to 'float' alters 'int' constant value

For the user, I think this is surprising and useless.

A more user-friendly approach would be to refer to the first location
that (in the reported macro expansion context) is for a location in
real source code, like what is shown in the new test case
gcc/testsuite/g++.dg/warn/Wconversion-real-integer2.C accompanying
this patch.

To do this, I am making the line-map module provide a new
linemap_unwind_to_first_non_reserved_loc function that resolves a
virtual location to the first spelling location that is in real source
code.

I am then using that facility in the diagnostics printing module and
in the macro unwinder to avoid printing diagnostics lines that refer
to the locations for built-ins or more generally for reserved
locations.  Note that when I start the dance of skipping a built-in
location I also skip locations that are in system headers, because it
turned out that a lot of those built-ins are actually used in system
headers (e.g, "#define INT_MAX __INT_MAX__" where __INT_MAX__ is a
built-in).

Besides the user-friendliness gain, this patch allows a number of
regression tests to PASS unchanged with and without
-ftrack-macro-expansion.

Tested and bootstrapped on x86_64-unknown-linux-gnu against trunk.

Note that the bootstrap with -ftrack-macro-expansion exhibits other
separate issues that are addressed in subsequent patches.  This patch
just fixes one class of problems.

The patch does pass bootstrap with -ftrack-macro-expansion turned off,
though.

libcpp/

	* include/line-map.h (linemap_unwind_toward_expansion): Fix typo
	in comment.
	(linemap_unwind_to_first_non_reserved_loc): Declare new function.
	* line-map.c (linemap_unwind_to_first_non_reserved_loc): Define
	new function.

gcc/

	* input.c (expand_location_1): When expanding to spelling location
	in a context of a macro expansion, skip reserved system header
	locations.  Update comments.  * tree-diagnostic.c
	(maybe_unwind_expanded_macro_loc): Likewise.

gcc/testsuite/

	* g++.dg/warn/Wconversion-real-integer2.C: New test.
	* g++.dg/warn/Wconversion-real-integer-3.C: Likewise.
	* g++.dg/warn/conversion-real-integer-3.h: New header used by the
	new test above.

From-SVN: r186970

libcpp/include/line-map.h

@@ -666,12 +666,30 @@ source_location linemap_resolve_location (struct line_maps *,
    location L of the point where M got expanded.  If L is a spelling
    location inside a macro expansion M', then this function returns
    the point where M' was expanded.  LOC_MAP is an output parameter.
-   When non-NULL, *LOC_MAP is set the the map of the returned
+   When non-NULL, *LOC_MAP is set to the map of the returned
    location.  */
 source_location linemap_unwind_toward_expansion (struct line_maps *,
 						 source_location loc,
 						 const struct line_map **loc_map);
 
+/* If LOC is the virtual location of a token coming from the expansion
+   of a macro M and if its spelling location is reserved (e.g, a
+   location for a built-in token), then this function unwinds (using
+   linemap_unwind_toward_expansion) the location until a location that
+   is not reserved and is not in a system header is reached.  In other
+   words, this unwinds the reserved location until a location that is
+   in real source code is reached.
+
+   Otherwise, if the spelling location for LOC is not reserved or if
+   LOC doesn't come from the expansion of a macro, the function
+   returns LOC as is and *MAP is not touched.
+
+   *MAP is set to the map of the returned location if the later is
+   different from LOC.  */
+source_location linemap_unwind_to_first_non_reserved_loc (struct line_maps *,
+							  source_location loc,
+							  const struct line_map **map);
+
 /* Expand source code location LOC and return a user readable source
    code location.  LOC must be a spelling (non-virtual) location.  If
    it's a location < RESERVED_LOCATION_COUNT a zeroed expanded source