Switch PPExpression parsing to be in terms of APInt's whose widths are properly

sized to the target's intmax_t.  This also allows us to emit diagnostic like:

t.c:11:5: warning: integer constant is too large for its type
#if 18446744073709551616  // 2^64
    ^
t.c:15:5: warning: integer constant is too large for its type
#if 18446744073709551617  // 2^64-1
    ^

llvm-svn: 39385