Introduce a Builtin.FixedArray type.

`Builtin.FixedArray<let N: Int, T: ~Copyable & ~Escapable>` has the layout of `N` elements of type `T` laid out
sequentially in memory (with the tail padding of every element occupied by the array). This provides a primitive
on which the standard library `Vector` type can be built.

include/swift/Runtime/RuntimeFunctions.def

@@ -1313,6 +1313,17 @@ FUNCTION(GetTupleLayout3, swift_getTupleTypeLayout3, SwiftCC, AlwaysAvailable,
          EFFECT(NoEffect),
          UNKNOWN_MEMEFFECTS)
 
+// MetadataResponse swift_getFixedArrayTypeMetadata(MetadataRequest request,
+//                                                  intptr_t size,
+//                                                  Metadata *elt);
+FUNCTION(GetFixedArrayTypeMetadata, swift_getFixedArrayTypeMetadata, SwiftCC,
+         ValueGenericTypeAvailability,
+         RETURNS(TypeMetadataResponseTy),
+         ARGS(SizeTy, SizeTy, TypeMetadataPtrTy),
+         ATTRS(NoUnwind, WillReturn),
+         EFFECT(MetaData),
+         MEMEFFECTS(ReadOnly))
+
 // Metadata *swift_getExistentialTypeMetadata(
 //                              ProtocolClassConstraint classConstraint,
 //                              const Metadata *superclassConstraint,