[clangd] Support extraction of binary "subexpressions" like a + [[b + c]].

Summary:
These aren't formally subexpressions in C++, in this case + is left-associative.
However informally +, *, etc are usually (mathematically) associative and users
consider these subexpressions.

We detect these and in simple cases support extracting the partial expression.
As well as builtin associative operators, we assume that overloads of them
are associative and support those too.

Reviewers: SureYeaah

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65139

llvm-svn: 367121