[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

@@ -336,16 +336,16 @@ class alignas(1 << TypeAlignInBits) TypeBase {
     NumProtocols : 16
   );
 
-  SWIFT_INLINE_BITFIELD_FULL(TypeVariableType, TypeBase, 64-NumTypeBaseBits,
+  SWIFT_INLINE_BITFIELD_FULL(TypeVariableType, TypeBase, 20+3+21,
     /// The unique number assigned to this type variable.
-    ID : 32 - NumTypeBaseBits,
+    ID : 20,
 
     /// Type variable options.
     Options : 3,
 
     ///  Index into the list of type variables, as used by the
     ///  constraint graph.
-    GraphIndex : 29
+    GraphIndex : 21
   );
 
   SWIFT_INLINE_BITFIELD(SILFunctionType, TypeBase, NumSILExtInfoBits+3+1+2,
@@ -5227,7 +5227,11 @@ class TypeVariableType : public TypeBase {
   TypeVariableType(const ASTContext &C, unsigned ID)
     : TypeBase(TypeKind::TypeVariable, &C,
                RecursiveTypeProperties::HasTypeVariable) {
+    // Note: the ID may overflow (current limit is 2^20 - 1).
     Bits.TypeVariableType.ID = ID;
+    if (Bits.TypeVariableType.ID != ID) {
+      llvm::report_fatal_error("Type variable id overflow");
+    }
   }
 
   class Implementation;
@@ -5263,6 +5267,10 @@ class TypeVariableType : public TypeBase {
     return reinterpret_cast<Implementation *>(this + 1);
   }
 
+  /// Type variable IDs are not globally unique and are
+  /// used in equivalence class merging (so representative
+  /// is always a type variable with smaller id), as well
+  /// as a visual aid when dumping AST.
   unsigned getID() const { return Bits.TypeVariableType.ID; }
 
   // Implement isa/cast/dyncast/etc.

lib/Sema/CSGen.cpp

@@ -360,7 +360,7 @@ namespace {
   /// expression, attempt to derive a favored type for it.
   bool computeFavoredTypeForExpr(Expr *expr, ConstraintSystem &CS) {
     LinkedTypeInfo lti;
-    
+
     expr->walk(LinkedExprAnalyzer(lti, CS));
 
     // Link anonymous closure params of the same index.
@@ -381,41 +381,21 @@ namespace {
       }
     }
 
-    // Link integer literal tyvars.
-    if (lti.intLiteralTyvars.size() > 1) {
-      auto rep1 = CS.getRepresentative(lti.intLiteralTyvars[0]);
-
-      for (size_t i = 1; i < lti.intLiteralTyvars.size(); i++) {
-        auto rep2 = CS.getRepresentative(lti.intLiteralTyvars[i]);
+    auto mergeTypeVariables = [&](ArrayRef<TypeVariableType *> typeVars) {
+      if (typeVars.size() < 2)
+        return;
 
+      auto rep1 = CS.getRepresentative(typeVars.front());
+      for (unsigned i = 1, n = typeVars.size(); i != n; ++i) {
+        auto rep2 = CS.getRepresentative(typeVars[i]);
         if (rep1 != rep2)
           CS.mergeEquivalenceClasses(rep1, rep2, /*updateWorkList*/ false);
       }
-    }
-
-    // Link float literal tyvars.
-    if (lti.floatLiteralTyvars.size() > 1) {
-      auto rep1 = CS.getRepresentative(lti.floatLiteralTyvars[0]);
-
-      for (size_t i = 1; i < lti.floatLiteralTyvars.size(); i++) {
-        auto rep2 = CS.getRepresentative(lti.floatLiteralTyvars[i]);
-        
-        if (rep1 != rep2)
-          CS.mergeEquivalenceClasses(rep1, rep2, /*updateWorkList*/ false);
-      }
-    }
-
-    // Link string literal tyvars.
-    if (lti.stringLiteralTyvars.size() > 1) {
-      auto rep1 = CS.getRepresentative(lti.stringLiteralTyvars[0]);
-
-      for (size_t i = 1; i < lti.stringLiteralTyvars.size(); i++) {
-        auto rep2 = CS.getRepresentative(lti.stringLiteralTyvars[i]);
+    };
 
-        if (rep1 != rep2)
-          CS.mergeEquivalenceClasses(rep1, rep2, /*updateWorkList*/ false);
-      }
-    }
+    mergeTypeVariables(lti.intLiteralTyvars);
+    mergeTypeVariables(lti.floatLiteralTyvars);
+    mergeTypeVariables(lti.stringLiteralTyvars);
 
     if (lti.collectedTypes.size() == 1) {
       // TODO: Compute the BCT.