locale: update ctype access for MSVC CRT 14+

Visual C++ 14 and newer split msvcrt into msvcrt and ucrt with flavours
of the ucrt for different environments.  This changed the access to the
ctype table by introducing the `__pctype_func` and `__pwctype_func`
accessors.  Use this rather than directly accessing `_ctype` which
allows us to be safer in threaded situations by going through the libc
locking.

llvm-svn: 290823