[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

llvm-svn: 244512