[FixedArray] Fix BitwiseCopyable inference.

`FixedArray<T> : BitwiseCopyable` iff `T : BitwiwseCopyable`;
determining this requires using `checkConformance`, not
`lookupConformance`.

Author: nate-chandler

lib/AST/Builtins.cpp

@@ -3385,11 +3385,12 @@ bool BuiltinType::isBitwiseCopyable() const {
     return false;
 
   case BuiltinTypeKind::BuiltinFixedArray: {
-    // Bitwise-copyability depends on the element type.
+    // FixedArray<N, X> : BitwiseCopyable whenever X : BitwiseCopyable
     auto bfa = cast<BuiltinFixedArrayType>(this);
     auto &C = bfa->getASTContext();
-    return (bool)lookupConformance(bfa->getElementType(),
-                           C.getProtocol(KnownProtocolKind::BitwiseCopyable));
+    return (bool)checkConformance(
+        bfa->getElementType(),
+        C.getProtocol(KnownProtocolKind::BitwiseCopyable));
   }
   }
 }

test/Sema/bitwise_copyable.swift

@@ -1,11 +1,16 @@
 // RUN: %target-typecheck-verify-swift                       \
+// RUN:     -disable-experimental-parser-round-trip          \
 // RUN:     -disable-availability-checking                   \
 // RUN:     -enable-experimental-feature NonescapableTypes   \
+// RUN:     -enable-experimental-feature ValueGenerics       \
 // RUN:     -enable-experimental-feature Sensitive           \
 // RUN:     -enable-builtin-module                           \
 // RUN:     -debug-diagnostic-names
 
+// FIXME: Remove -disable-experimental-parser-round-trip when it's not required for using ValueGenerics.
+
 // REQUIRES: swift_feature_NonescapableTypes
+// REQUIRES: swift_feature_ValueGenerics
 // REQUIRES: swift_feature_Sensitive
 
 //==============================================================================
@@ -109,6 +114,17 @@ func passS_Implicit_Sensitive(_ s: S_Implicit_Sensitive) {
            // expected-note@-94 {{where_requirement_failure_one_subst}}
 }
 
+import Builtin
+
+func passFixedArray1N<T>(_ fa: Builtin.FixedArray<1, T>) {
+  take1(fa) // expected-error {{type_does_not_conform_decl_owner}}
+            // expected-note@-101 {{where_requirement_failure_one_subst}}
+}
+
+func passFixedArray1N<T : BitwiseCopyable>(_ fa: Builtin.FixedArray<1, T>) {
+  take1(fa)
+}
+
 //==============================================================================
 //===========================DEPENDENCY-FREE TESTS=(END)======================}}
 //==============================================================================