Fix -Wuninitialized for -ftrack-macro-expansion

Fix -Wuninitialized for -ftrack-macro-expansion

Besides the warning emitted by warn_uninit, this function wants
to hint the user at where the uninitialized variable was declared, for
cases where the declaration location is outside the current function.

Now that expand_location expands to the location that is in the main
source file (even for -ftrack-macro-expansion) the hinting part was
not working well for cases where the variable is declared in a macro
(outside the function), which is then expanded in the function.

So I had to adjust warn_uninit a little bit to make it consider the
spelling location of the variable declaration.

I have fixed the test gcc.dg/cpp/pragma-diagnostic-2.c on which I
believe gcc shouldn't emit any error.

Here is the new output on that test:

=~=
gcc.dg/cpp/pragma-diagnostic-2.c: In function 'g':
gcc.dg/cpp/pragma-diagnostic-2.c:10:5: warning: 'a' is used uninitialized in this function [-Wuninitialized]
gcc.dg/cpp/pragma-diagnostic-2.c:9:7: note: 'a' was declared here
gcc.dg/cpp/pragma-diagnostic-2.c:9:7: note: in expansion of macro 'CODE_WITH_WARNING'
gcc.dg/cpp/pragma-diagnostic-2.c:17:3: note: expanded from here
gcc.dg/cpp/pragma-diagnostic-2.c: In function 'h':
gcc.dg/cpp/pragma-diagnostic-2.c:10:5: warning: 'a' is used uninitialized in this function [-Wuninitialized]
gcc.dg/cpp/pragma-diagnostic-2.c:9:7: note: 'a' was declared here
gcc.dg/cpp/pragma-diagnostic-2.c:9:7: note: in expansion of macro 'CODE_WITH_WARNING'
gcc.dg/cpp/pragma-diagnostic-2.c:27:3: note: expanded from here
=~=

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

Note that the bootstrap with -ftrack-macro-expansion turned on
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.

gcc/
	* tree-ssa.c (warn_uninit): Use the spelling location of the
	variable declaration.  Use linemap_location_before_p for source
	locations.

gcc/testsuite/

	* gcc.dg/cpp/pragma-diagnostic-2.c:  Fix this.

From-SVN: r186971