[Sema] Avoid repeated hash lookups (NFC) (#109375)
GlobalMethodPool, the type of MethodPool, is a type wrapping DenseMap
and exposes only a subset of the DenseMap methods.
This patch adds operator[] to GlobalMethodPool so that we can avoid
repeated hash lookups. I don't bother using references or rvalue
references in operator[] because Selector, the key type, is small and
trivially copyable. Note that Selector is a class that wraps a
PointerUnion.
