[index] When indexing an ObjC method declaration use its base name for the location.

[index] When indexing an ObjC method declaration use its base name for the location.

Instead of using the location of the beginning '-'/'+'.
This is consistent with location used for function decls and ObjC method calls where we use the base name as the location as well.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@293134 91177308-0d34-0410-b5e6-96231b3b80d8