[WebKit checkers] Recognize ensureFoo functions (#119681)
In WebKit, we often write Foo::ensureBar function which lazily
initializes m_bar and returns a raw pointer or a raw reference to m_bar.
Such a return value is safe to use for the duration of a member function
call in Foo so long as m_bar is const so that it never gets unset or
updated with a new value once it's initialized.
This PR adds support for recognizing these types of functions and
treating its return value as a safe origin of a function argument
(including "this") or a local variable.
