[llvm-demangle-fuzzer] Also fuzz microsoftDemangle().

Summary:
Use first byte of input to determine whether to call itaniumDemangle()
or microsoftDemangle().

Addresses https://bugs.llvm.org/show_bug.cgi?id=39582.

Reviewers: kcc, thakis

Reviewed By: kcc, thakis

Subscribers: mgorny, thakis, erik.pilkington, llvm-commits

Differential Revision: https://reviews.llvm.org/D54780

llvm-svn: 350534