[llvm-symbolizer] Add -C as a short alias to -demangle

Summary: Provides -C as alias to -demangle. Motivation: https://bugs.llvm.org/show_bug.cgi?id=40069.

Reviewers: jhenderson, ruiu, rnk, fjricci

Reviewed By: jhenderson, ruiu

Subscribers: rupprecht, erik.pilkington, llvm-commits

Differential Revision: https://reviews.llvm.org/D56591

llvm-svn: 351300