Track overflow of shift amounts, allowing diagnostics like:

t.c:6:7: warning: integer overflow in preprocessor expression
#if 1 << 63
      ^
t.c:8:7: warning: integer overflow in preprocessor expression
#if 4 << 62
      ^
t.c:10:7: warning: integer overflow in preprocessor expression
#if 4 << 66
      ^

but no diagnostic on:
#if 1U << 63

llvm-svn: 39400