[llvm-cov] Fix llvm-cov on Windows and un-XFAIL test

Summary:
The llvm-cov tool needs to be able to find coverage names in the
executable, so the .lprfn and .lcovmap sections cannot be merged into
.rdata.

Also, the linker merges .lprfn$M into .lprfn, so llvm-cov needs to
handle that when looking up sections. It has to support running on both
relocatable object files and linked PE files.

Lastly, when loading .lprfn from a PE file, llvm-cov needs to skip the
leading zero byte added by the profile runtime.

Reviewers: vsk

Subscribers: hiraditya, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D58661

llvm-svn: 354840