[SE-0458] Disambiguate "unsafe" expression and for..in effect

Port over the disambiguation code we already had in the new Swift parser
to the existing C++ parser for the "unsafe" expression and for..in effect.

Author: DougGregor

lib/Parse/ParseExpr.cpp

@@ -436,7 +436,9 @@ ParserResult<Expr> Parser::parseExprSequenceElement(Diag<> message,
     return sub;
   }
 
-  if (Tok.isContextualKeyword("unsafe")) {
+  if (Tok.isContextualKeyword("unsafe") &&
+      !peekToken().isAtStartOfLine() &&
+      !peekToken().is(tok::r_paren)) {
     Tok.setKind(tok::contextual_keyword);
     SourceLoc unsafeLoc = consumeToken();
     ParserResult<Expr> sub =

lib/Parse/ParseStmt.cpp

@@ -2379,7 +2379,8 @@ ParserResult<Stmt> Parser::parseStmtForEach(LabeledStmtInfo LabelInfo) {
     }
   }
 
-  if (Tok.isContextualKeyword("unsafe")) {
+  if (Tok.isContextualKeyword("unsafe") &&
+      !peekToken().isAny(tok::colon, tok::kw_in)) {
     UnsafeLoc = consumeToken();
   }
 

test/Unsafe/safe.swift

@@ -98,6 +98,15 @@ func testUnsafeAsSequenceForEach() {
   for unsafe _ in unsafe uas { } // okay
 }
 
+func testForInUnsafeAmbiguity(_ integers: [Int]) {
+  for unsafe in integers {
+    _ = unsafe
+  }
+  for unsafe: Int in integers {
+    _ = unsafe
+  }
+}
+
 struct UnsafeIterator: @unsafe IteratorProtocol {
   @unsafe mutating func next() -> Int? { nil }
 }