[asan] Instrument comdat globals on COFF targets

Summary:
If we can use comdats, then we can make it so that the global metadata
is thrown away if the prevailing definition of the global was
uninstrumented. I have only tested this on COFF targets, but in theory,
there is no reason that we cannot also do this for ELF.

This will allow us to re-enable string merging with ASan on Windows,
reducing the binary size cost of ASan on Windows.

Reviewers: eugenis, vitalybuka

Subscribers: hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D47841

llvm-svn: 334313