[Statement checker] Simplify checkUnknownAttrRestrictions().

This routine can compute the fallthrough destination on its own. When
it does, we start correctly checking @unknown cases in function
builders, so update and expand the test accordingly.

Author: DougGregor

lib/Sema/BuilderTransform.cpp

@@ -1348,8 +1348,7 @@ class BuilderClosureRewriter
       // Check restrictions on '@unknown'.
       if (caseStmt->hasUnknownAttr()) {
         checkUnknownAttrRestrictions(
-            cs.getASTContext(), caseStmt, /*fallthroughDest=*/nullptr,
-            limitExhaustivityChecks);
+            cs.getASTContext(), caseStmt, limitExhaustivityChecks);
       }
 
       ++caseIndex;

lib/Sema/TypeCheckStmt.cpp

@@ -1236,8 +1236,7 @@ class StmtChecker : public StmtVisitor<StmtChecker, Stmt*> {
         assert(parentKind == CaseParentKind::Switch &&
                "'@unknown' can only appear on switch cases");
         checkUnknownAttrRestrictions(
-            getASTContext(), caseBlock, FallthroughDest,
-            limitExhaustivityChecks);
+            getASTContext(), caseBlock, limitExhaustivityChecks);
       }
 
       Stmt *body = caseBlock->getBody();
@@ -2157,8 +2156,9 @@ void TypeChecker::typeCheckTopLevelCodeDecl(TopLevelCodeDecl *TLCD) {
 }
 
 void swift::checkUnknownAttrRestrictions(
-    ASTContext &ctx, CaseStmt *caseBlock, CaseStmt *fallthroughDest,
+    ASTContext &ctx, CaseStmt *caseBlock,
     bool &limitExhaustivityChecks) {
+  CaseStmt *fallthroughDest = caseBlock->findNextCaseStmt();
   if (caseBlock->getCaseLabelItems().size() != 1) {
     assert(!caseBlock->getCaseLabelItems().empty() &&
            "parser should not produce case blocks with no items");

lib/Sema/TypeChecker.h

@@ -1377,8 +1377,7 @@ bool areGenericRequirementsSatisfied(const DeclContext *DC,
 /// Check for restrictions on the use of the @unknown attribute on a
 /// case statement.
 void checkUnknownAttrRestrictions(
-    ASTContext &ctx, CaseStmt *caseBlock, CaseStmt *fallthroughDest,
-    bool &limitExhaustivityChecks);
+    ASTContext &ctx, CaseStmt *caseBlock, bool &limitExhaustivityChecks);
 
 /// Bind all of the pattern variables that occur within a case statement and
 /// all of its case items to their "parent" pattern variables, forming chains

test/Constraints/function_builder_diags.swift

@@ -433,16 +433,27 @@ struct TestConstraintGenerationErrors {
 
 // Check @unknown
 func testUnknownInSwitchSwitch(e: E) {
-    tuplify(true) { c in
+  tuplify(true) { c in
     "testSwitch"
     switch e {
-    @unknown case .a: // expected-error{{'@unknown' is only supported for catch-all cases ("case _")}}
-      "a"
     case .b(let i, let s?):
       i * 2
       s + "!"
     default:
       "nothing"
+    @unknown case .a: // expected-error{{'@unknown' is only supported for catch-all cases ("case _")}}
+      // expected-error@-1{{'case' blocks cannot appear after the 'default' block of a 'switch'}}
+      "a"
+    }
+  }
+
+  tuplify(true) { c in
+    "testSwitch"
+    switch e {
+      @unknown case _: // expected-error{{'@unknown' can only be applied to the last case in a switch}}
+      "a"
+    default:
+      "default"
     }
   }
 }