[AST] Increase size of type variable id field to 20 bits

[xedin]

Currently id field is only 12 bits long which is a lot smaller
than graph index which is 28 bits and creates problems with
large collections of literals since id is going to overflow
pretty quickly (4096 elements max). Let's change it so both
fields have the same 20 bits which is big enough.

Resolves: SR-10914
Resolves: rdar://problem/51616307

[xedin]

@swift-ci please test

[jckarter]

Looks good to me.

[xedin]

@swift-ci please test

[xedin]

@swift-ci please test

[xedin]

@swift-ci please test source compatibility

[xedin]

@swift-ci please smoke test

[davezarzycki]

This change makes the TypeVariableType bitfield very sensitive to TypeBase bitfield experiments. Can we add a comment suggesting what one should do? Perhaps dial back the graph index to 19? If that's the advice, can we just automate it? For example:

@@ -341,18 +341,19 @@ protected:
     : NumPadBits,
     NumProtocols : 16
   );
 
-  SWIFT_INLINE_BITFIELD_FULL(TypeVariableType, TypeBase, 20+4+20,
+  SWIFT_INLINE_BITFIELD_FULL(TypeVariableType, TypeBase, 20+4,
     /// The unique number assigned to this type variable.
     ID : 20,
 
     /// Type variable options.
     Options : 4,
 
     ///  Index into the list of type variables, as used by the
     ///  constraint graph.
-    GraphIndex : 20
+    GraphIndex : NumPadBits;
+    static_assert(NumPadBits >= 16, "TypeVariableType bitfield needs refactoring")
   );
 
   SWIFT_INLINE_BITFIELD(SILFunctionType, TypeBase, NumSILExtInfoBits+3+1+2,
     ExtInfo : NumSILExtInfoBits,

[davezarzycki]

See #25818