Handle name prefixing in sanitizeGlobal/sanitizeLocal.

In the case of sanitizeGlobal, "_" was being prefixed in two different
places; this factors it out into one place.
Simplify the handling of unnamed values.

This will also make it easier for the upcoming AllocaManager to eliminate
allocas, because unnamed eliminated allocas won't be assigned names.
Introduce AllocaManager, a class for managing allocas.

The AllocaManager performs stack layout for alloca objects. It is capable
of analyzing alloca liveness and sharing space between multiple allocas.
Perform alloca coloring only at -O1 and higher.
Don't run SimplifyAllocas at -O1 and higher.
Don't run calculateNativizedVars at -O1 and higher.