Debug Info: Relax assertion in isUnsignedDIType() to allow floats to be

described by integer constants. This is a bit ugly, but if the source
language allows arbitrary type casting, the debug info must follow suit.

For example:
  void foo() {
    float a;
    *(int *)&a = 0;
  }
For the curious: SROA replaces the float alloca with an i32 alloca, which
is then optimized away and described via dbg.value(i32 0, ...).

llvm-svn: 227827