[Sema] Declare GlobalMethodPool with using (NFC) (#109437)
GlobalMethodPool is a wrapper around DenseMap that does not add
anything except:
using Lists = std::pair<ObjCMethodList, ObjCMethodList>;
This patch removes the wrapper and switches to an alias with "using".
In ReadMethodPool in ASTReader.cpp, we can simplify:
insert(std::make_pair(Sel, SemaObjC::GlobalMethodPool::Lists()))
to:
try_emplace(Sel)
But then try_emplace(Sel).first->second is the same as operator[], so
this patch simplifies the rest of the function.
