NFC: repack decl base

[davezarzycki]

Take advantage of unused bits in the Decl base class to repack subclass bits to save memory.

[davezarzycki]

@swift-ci please test and merge

[davezarzycki]

@swift-ci please smoke test and merge

[davezarzycki]

@swift-ci please test and merge

[davezarzycki]

@swift-ci please test and merge

[jrose-apple]

Does this actually save memory? Decl subclasses that don't need more than 32 bits are now paying for it.

[davezarzycki]

For various reasons, yes.

1. If one reviews the SCM history, there was a comment that the Decl base had 4 bytes of wasted padding after the bitfield.
2. If we set the comment aside, natural alignment rules and the frequent use of pointers in data structures tends to make the vast majority of types be 64-bit aligned/padded on 64-bit architectures.
3. Finally, computer vendors haven't sold 32-bit only workstations in many years. Therefore trying to optimize for 32-bit environments is fairly pointless.
4. I've been using -Wpadded to verify memory savings.

[jrose-apple]

Ah, if we didn't have anything else that could fit in those 32 bits, then this certainly makes sense. Okay, thanks, Dave!