Allow a pointer implicitly cast to a bool as a constant expression, as

Allow a pointer implicitly cast to a bool as a constant expression, as
required by the standard (the standard doesn't know anything about
implicit casts).

Disallow pointers cast to non-integral arithmetic types as constant
expressions.  This was previously allowed by accident.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@51779 91177308-0d34-0410-b5e6-96231b3b80d8