[clang][ASTMatcher] Add matchers for isExplicitObjectMemberFunction() (#84446)

Note that this patch will be necessary to fix
`forEachArgumentWithParam()` and `forEachArgumentWithParamType()`
matchers for deducing "this"; which is my true motivation. There the bug
is that with explicit obj params, one should not adjust the number of
arguments in presence of `CXXMethodDecls`, and this causes a mismatch
there mapping the argument to the wrong param. But, I'll come back there
once we have this matcher.