Look through "unsafe" expressions when checking for single-value statements

We were rejecting the use of switch expressions on the right-hand side
of an assignment that was marked `unsafe`. Fixes rdar://147944753.

Author: DougGregor

lib/Sema/PreCheckTarget.cpp

@@ -1680,6 +1680,11 @@ void PreCheckTarget::markAnyValidSingleValueStmts(Expr *E) {
       while (auto *IIO = dyn_cast<InjectIntoOptionalExpr>(E))
         E = IIO->getSubExpr();
     }
+
+    // Look through "unsafe" expressions.
+    if (auto UE = dyn_cast<UnsafeExpr>(E))
+      E = UE->getSubExpr();
+
     return dyn_cast<AssignExpr>(E);
   };
 

test/Unsafe/safe.swift

@@ -241,3 +241,11 @@ func testMyArray(ints: MyArray<Int>) {
     unsafe print(buffer.unsafeCount)
   }
 }
+
+func testUnsafeLHS() {
+  @unsafe var value: Int = 0
+  unsafe value = switch unsafe value {
+  case 0: 1
+  default: 0
+  }
+}