MS ABI: Simplify MangleByte further

It turns out that the ranges where the '?' <letter> manglings occur are
identical to the ranges of ASCII characters OR'd with 0x80.

Thanks to Richard Smith for the insight!

No functional change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205270 91177308-0d34-0410-b5e6-96231b3b80d8