[Clangd] Fixed ExtractVariable for certain types of Exprs

Summary:

- Modified ExtractVariable for extraction of MemberExpr, DeclRefExpr and Assignment Expr
- Removed extraction from label statements.
- Fixed unittests

Reviewers: sammccall, kadircet

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64717

llvm-svn: 366869