Catch overflow of negate. This allows us to emit:

t.c:2:17: warning: integer overflow in preprocessor expression
#if -(INTMAX_MAX+1)
                ^
t.c:2:5: warning: integer overflow in preprocessor expression
#if -(INTMAX_MAX+1)
    ^

GCC has some seriously confused source locations in this case:
t.c:2:19: warning: integer overflow in preprocessor expression
t.c:2:20: warning: integer overflow in preprocessor expression

column 19/20 are off the end of the line.

llvm-svn: 39404