Previously SwitchEnumBuilder was in SILGenBuilder.{h,cpp} and
SwitchEnumCaseFullExpr was in its own file. This really made no sense since:
1. The two classes are inherently related to each other so really should be
together in the source base.
2. SwitchEnumBuilder uses a SILGenBuilder, but is really a separate independent
concept/entity, so there really is no reason to keep it in SILGenBuilder.cpp.
Just a quick fix to eliminate something that was bugging me.
NFC.