[libclang 7/8] Add support for getting property setter and getter names

Summary: This allows libclang to access the actual names of property setters and getters without needing to go through the indexer API. Usually default names are used, but the property can specify a different name.

Reviewers: yvvan, jbcoe

Reviewed By: yvvan

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D49634

llvm-svn: 338816