[libclang] indexing: Have the semantic container of synthesized ObjC...

[libclang] indexing: Have the semantic container of synthesized ObjC getter/setter methods be the implementation decl.

Matches the behavior of other ObjC methods.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@260250 91177308-0d34-0410-b5e6-96231b3b80d8