[clang][ASTImporter] Improve structural equivalence of overloadable operators. (#72242)

Operators that are overloadable may be parsed as `CXXOperatorCallExpr`
or as `UnaryOperator` (or `BinaryOperator`). This depends on the context
and can be different if a similar construct is imported into an existing
AST. The two "forms" of the operator call AST nodes should be detected
as equivalent to allow AST import of these cases.

This fix has probably other consequences because if a structure is
imported that has `CXXOperatorCallExpr` into an AST with an existing
similar structure that has `UnaryOperator` (or binary), the additional
data in the `CXXOperatorCallExpr` node is lost at the import (because
the existing node will be used). I am not sure if this can cause
problems.