[NFC][analyzer] Rename `CheckerBase::getCheckerName` to `getName` (#130953)

The method name `getCheckerName` would imply "get the name of the
checker associated with `this`", so it's suitable for e.g.
`BugType::getCheckerName` -- but a method that just "gets the name of
`this`" should be simply called `getName`.

This change eliminates the redundant and ugly pattern
`Checker->getCheckerName()` and helps to visually distinguish this
method from `BugType::getCheckerName`.

In the constructor of `BugType` the call of this method was completely
removed (instead of just changing the name) because that call was dead
code (when the data member `Checker` is non-null, the string stored in
`CheckerName` is irrelevant) and was often querying the name of the
checker before it was properly initialized.

Moreover, in `ReturnValueChecker.cpp` the static helper function
`getName` (which gets a function name from a `CallEvent`) was renamed to
the more specific `getFunctionName` to avoid the name collision.

This change is yet another cleanup commit before my planned changes that
would add support for multi-part checkers to this method.