[libc++] Clean up cl warning 4231 disabling

Once upon a time, extern templates used to be a Microsoft extension, so
cl would warn about their usage, and libc++ suppressed that warning.
They've long since been standardized, so the warning is defunct. (libc++
also doesn't currently support building with cl anyway.)

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@307997 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed