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

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](https://bugs.swift.org/browse/SR-10914)
Resolves: rdar://problem/51616307

Author: xedin

include/swift/AST/Types.h

@@ -338,14 +338,14 @@ class alignas(1 << TypeAlignInBits) TypeBase {
 
   SWIFT_INLINE_BITFIELD_FULL(TypeVariableType, TypeBase, 64-NumTypeBaseBits,
     /// The unique number assigned to this type variable.
-    ID : 32 - NumTypeBaseBits,
+    ID : 20,
 
     /// Type variable options.
     Options : 4,
 
     ///  Index into the list of type variables, as used by the
     ///  constraint graph.
-    GraphIndex : 28
+    GraphIndex : 20
   );
 
   SWIFT_INLINE_BITFIELD(SILFunctionType, TypeBase, NumSILExtInfoBits+3+1+2,