Use the APFloat routines to evaluate FP immediates as

integer constant expressions.  The only questionable
thing is that we now reject:

void foo() {
  switch (1) {
  case (int)1.0e10000:
    ;
  }
}

with:

t.c:5:13: error: case label does not reduce to an integer constant
  case (int)1.0e10000:
       ~~~~~^~~~~~~~~

GCC accepts this, emitting the pedwarn:
t.c:5: warning: floating constant exceeds range of 'double'

llvm-svn: 42238