[Clang][Type] Add static assertion to guard future expansion for BuiltinType numbering

[eopXD]

Summary

Recent expansion D152070 exceeds the limit and I had a hard time
triaging the bug because the overflow just creates unexpected
behaviors within the compiler.

This patch adds a static assertion to keep an eye for overflows when we
expand more types in the future.

[aaron.ballman]

Thanks, this generally looks good but I did spot a small change to make (can make it when landing).

clang/include/clang/AST/Type.h

1542

I think this should move down to be directly above the definition of BuiltinTypeBitfields.

[eopXD]

Address comment from Aaron.

clang/include/clang/AST/Type.h

enum class AutoTypeKeyword {
 
   /// decltype(auto)
   DecltypeAuto,
 
   /// __auto_type (GNU extension)
   GNUAutoType
 };
 
+static const unsigned NumOfBuiltinTypeBits = 8;
+
 /// The base class of the type hierarchy.
 ///
 /// A central concept with types is that each type always has a canonical
 /// type.  A canonical type is the type with any typedef names stripped out
 /// of it or the types it references.  For example, consider:
 ///
 ///  typedef int  foo;
 ///  typedef foo* bar;
protected:
   };
 
   class BuiltinTypeBitfields {
     friend class BuiltinType;
 
     unsigned : NumTypeBits;
 
     /// The kind (BuiltinType::Kind) of builtin type this is.
-    unsigned Kind : 8;
+    unsigned Kind : NumOfBuiltinTypeBits;
   };
 
   /// FunctionTypeBitfields store various bits belonging to FunctionProtoType.
   /// Only common bits are stored here. Additional uncommon bits are stored
   /// in a trailing object after FunctionProtoType.
   class FunctionTypeBitfields {
     friend class FunctionProtoType;
     friend class FunctionType;

 
 private:
   friend class ASTContext; // ASTContext creates these.
 
   BuiltinType(Kind K)
       : Type(Builtin, QualType(),
              K == Dependent ? TypeDependence::DependentInstantiation
                             : TypeDependence::None) {
+    static_assert(Kind::LastKind < (1 << NumOfBuiltinTypeBits) &&
+                  "Defined builtin type exceeds the allocated space for serial "
+                  "numbering");
     BuiltinTypeBits.Kind = K;
   }
 
 public:
   Kind getKind() const { return static_cast<Kind>(BuiltinTypeBits.Kind); }
   StringRef getName(const PrintingPolicy &Policy) const;
 
   const char *getNameAsCString(const PrintingPolicy &Policy) const {