This diff will de-duplicate methods read from AST files before
inserting them in to a global method pool list. When reading
ObjCMethodDecl from AST files we can end up with a significant
amount of duplication when modules contain redeclarations of
system framework methods. For instance a common pattern is to
redeclare -(instancetype)init with NS_UNAVAILABLE, which
results in the entire ObjCMethodList for init being serialized
in each module with this redeclaration.
Measuring this against our codebase for files that use -fmodules
shows an overall 19% compile time improvement, and in some cases
as much as 79% for files with a lot of modular dependencies.