Correctly represent and propagate signedness information in preprocessor

constant expressions.  This allows us to emit this diagnostic:

t.c:5:5: warning: integer constant is so large that it is unsigned
#if 12345678901234567890
    ^

And makes constant evaluation fully correct, but we do not yet detect and
warn about integer overflow.

This patch requires cvs up'ing the main llvm tree to get the APSInt class,
but no libraries need to be rebuilt there.

llvm-svn: 39388