swift234

IRGen: Improve EnumImplStrategy's layout querying functionality with feedback from Enrico.

Only claim spare bits we actually use as tag bits in the result of getTagBitsForPayloadCases(), and fix up some crashers in corner cases. Add some assertions so these APIs get tested as part of normal compiler runs, and some DEBUG() macros so interested compiler users can see what enum layout is doing with -debug-only=enum-layout. This should also lead to slightly better codegen for some multi-payload enums because we don't pointlessly try to gather spare bits from the entire payload anymore, only the bits we actually need to discern the tag.

Swift SVN r12314