[llvm-cxxfilt] Correctly demangle COFF import thunk

Summary:
llvm-cxxfilt wasn't correctly demangle COFF import thunk in those two
cases before:
* demangle in split mode (multiple words from commandline)
* the import thunk prefix was added no matter the later part of the
string can be demangled or not
Now llvm-cxxfilt should handle both case correctly.

Reviewers: compnerd, erik.pilkington, jhenderson

Reviewed By: jhenderson

Subscribers: jkorous, dexonsmith, ributzka, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71425