Only consider built-in compound assignment operators for -Wunused-but-set-*

At least LibreOffice has, for mainly historic reasons that would be hard to
change now, a class Any with an overloaded operator >>= that semantically does
not assign to the LHS but rather extracts into the (by-reference) RHS.  Which
thus caused false positive -Wunused-but-set-parameter and
-Wunused-but-set-variable after those have been introduced recently.

This change is more conservative about the assumed semantics of overloaded
operators, excluding compound assignment operators but keeping plain operator =
ones.  At least for LibreOffice, that strikes a good balance of not producing
false positives but still finding lots of true ones.

(The change to the BinaryOperator case in MaybeDecrementCount is necessary
because e.g. the template f4 test code in warn-unused-but-set-variables-cpp.cpp
turns the += into a BinaryOperator.)

Differential Revision: https://reviews.llvm.org/D103949

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