Rework the C strings tests to use ASSERT_SAME_TYPE. NFC there. Also change...

Rework the C strings tests to use ASSERT_SAME_TYPE. NFC there. Also change cwchar.pass.cpp to avoid constructing a couple things from zero - since apparently they can be enums in some weird C library. NFC there, either, since the values were never used.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@349522 91177308-0d34-0410-b5e6-96231b3b80d8