[SanitizerCoverage] Don't duplicate code to get section pointers

Summary:
Merge code used to get section start and section end pointers
for SanitizerCoverage constructors. This includes code that handles
getting the start pointers when targeting MSVC.

Reviewers: kcc, morehouse

Reviewed By: morehouse

Subscribers: kcc, hiraditya

Differential Revision: https://reviews.llvm.org/D53211

llvm-svn: 344657