[CodeComplete] Don't skip single-expression AssignExpr bodies of an if/switch expr

Fix an issue uncovered by the stress tester where
the brace element skipping logic could still attempt
to skip a single-expression body of an if/switch
expr.

Author: hamishknight

lib/Sema/CSSyntacticElement.cpp

@@ -1141,9 +1141,13 @@ class SyntacticElementConstraintGenerator
 
     for (auto element : braceStmt->getElements()) {
       if (cs.isForCodeCompletion() &&
-          !cs.containsIDEInspectionTarget(element)) {
+          !cs.containsIDEInspectionTarget(element) &&
+          !(braceStmt->getSingleActiveExpression() &&
+            locator->isForSingleValueStmtConjunctionOrBrace())) {
         // To improve performance, skip type checking elements that can't
-        // influence the code completion token.
+        // influence the code completion token. Note we don't do this for
+        // single expression SingleValueStmtExpr branches, as they're needed to
+        // infer the type.
         if (element.is<Stmt *>() && !element.isStmt(StmtKind::Guard) && !element.isStmt(StmtKind::Return)) {
           // Statements can't influence the expresion that contains the code
           // completion token.

test/IDE/complete_if_switch_expr.swift

@@ -335,6 +335,14 @@ func testSkipTypeChecking14() -> E {
   }
 }
 
+func testSkipTypechecking15(_ x: inout Int) -> E {
+  switch Bool.random() {
+  case true:
+    .#^DOT27?check=DOT^#
+  case false:
+    x = 0
+  }
+}
 
 // DOT:     Begin completions, 2 items
 // DOT-DAG: Decl[EnumElement]/CurrNominal/Flair[ExprSpecific]/TypeRelation[Convertible]: e[#E#]; name=e