[clangd] Added an early return from VisitMemberExpr in SemanticHighlighting if...

[clangd] Added an early return from VisitMemberExpr in SemanticHighlighting if underlying MemberDecl is a CXXConversionDecl.

Summary:
Conversion operators contain invalid MemberLocs which caused SemanticHighlighting
to emit a lot of error logs in large files as they can occur fairly
often (for example converting StringRef to std string).
As the only thing happening was a lot of error logs being
emited there doesn't really seem to be any way to test this
(no erroneous tokens are added). But emiting as many logs as
were being emited is not wanted.

This also adds a test to guard against regressions for highlightings
disapearing from places where the conversion operators are used as their
behaviour differ from the other CXXMethodDecls.

Reviewers: hokein, ilya-biryukov

Subscribers: MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65928

llvm-svn: 368287