[clangd] Disallow extraction of expression-statements.
Summary:
I split out the "extract parent instead of this" logic from the "this isn't
worth extracting" logic (now in eligibleForExtraction()), because I found it
hard to reason about.
While here, handle overloaded as well as builtin assignment operators.
Also this uncovered a bug in getCallExpr() which I fixed.
Reviewers: SureYeaah
Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, cfe-commits
Tags: #clang
Differential Revision: https://reviews.llvm.org/D65337
llvm-svn: 368500