The conformance existing isn't enough to be sure the cast will succeed,
there may be dynamic information, so we just assume they're always
dynamic and always "MaySucceed".
Fixes rdar://problem/38694450.
Local.cpp was ~3k lines of which 1.5k (i.e. 1/2) was the cast optimizer. This
commit extracts the cast optimizer into its own .cpp and .h file. It is large
enough to stand on its own and allows for Local.cpp to return to being a small
group of helper functions.
I am making some changes in this area due to the change in certain function
conventions caused by the +0-normal-arg work. I am just trying to leave the area
a little cleaner than before.
