[llvm-symbolizer] Remove underscores and other C mangling on Windows

[llvm-symbolizer] Remove underscores and other C mangling on Windows

Summary:
This makes it so that reports symbolized after the fact with
llvm-symbolizer are more similar to the ones we generate at runtime with
in-process dbghelp.

Reviewers: samsonov

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11785

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@244512 91177308-0d34-0410-b5e6-96231b3b80d8