MS ABI: Simplify MangleByte

The delta between '\xe1' and '\xc1' is equivalent to the one between 'a'
and 'A'.  This allows us to reuse the computation between '\xe1' and
'\xfa' for the '\xc1' to '\xda' case.

No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205128 91177308-0d34-0410-b5e6-96231b3b80d8