swift234

Devirtualize during mandatory inlining.

With this change we will devirtualize in trivial cases where mandatory
inlining has exposed opportunities due to substituting types, for
example substituting a struct type into a witness_method where we can
now easily determine exactly what method will be called.

This makes it possible to use @transparent on struct methods that are
dispatched via generic functions, resulting in the opportunity to emit
diagnostics for these methods as well as eliminate the overhead of the
indirect call.

I saw a handful of 10+% perf improvements at -Onone on our benchmarks.

In theory this should allow us to remove the overloads for ++/-- in
FixedPoint.swift.gyb without a performance penalty (and with the proper
overflow diagnostics), but unfortunately if we were to do so, we would
currently dispatch to functions that lack runtime overflow
checks (rdar://problem/20226526).

Swift SVN r26397