Disallow reinterpret_cast from pointer to bool on Windows

This became allowed by accident in r131201, but triggers an assert.
That patch added an exception to allow conversion from pointers to
narrow integral types for MSVC compatibility. However, a pointer can
already be converted to bool in a civilized manner; allowing conversion
via reinterpret_cast is a bad idea.

Fixes PR16222.

llvm-svn: 183394