[Diagnostics] Adjust OoO fix-it intervals to account for replacement of first argument

Current logic fix-it didn't account for the first argument being re-ordered.
The start position for the first argument should always be the next token
after left paren denoting a start of the argument list.

Resolves: rdar://problem/70764991
(cherry picked from commit 09f3461)

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -4812,10 +4812,19 @@ bool OutOfOrderArgumentFailure::diagnoseAsError() {
     auto text = SM.extractText(
         Lexer::getCharSourceRangeFromSourceRange(SM, firstRange));
 
-    auto removalRange =
-        SourceRange(Lexer::getLocForEndOfToken(
-                        SM, tuple->getElement(ArgIdx - 1)->getEndLoc()),
-                    firstRange.End);
+    SourceLoc removalStartLoc;
+    // For the first argument, start is always next token after `(`.
+    if (ArgIdx == 0) {
+      removalStartLoc = tuple->getLParenLoc();
+    } else {
+      // For all other arguments, start is the next token past
+      // the previous argument.
+      removalStartLoc = tuple->getElement(ArgIdx - 1)->getEndLoc();
+    }
+
+    SourceRange removalRange{Lexer::getLocForEndOfToken(SM, removalStartLoc),
+                             firstRange.End};
+
     diag.fixItRemove(removalRange);
     diag.fixItInsert(secondRange.Start,
                      text.str() + (isExpr<BinaryExpr>(anchor) ? "" : ", "));

test/Constraints/argument_matching.swift

@@ -1745,3 +1745,17 @@ func --- (_ lhs: String, _ rhs: String) -> Bool { true }
 
 let x = 1
 x --- x // expected-error 2 {{cannot convert value of type 'Int' to expected argument type 'String'}}
+
+// rdar://problem/70764991 - out-of-order diagnostic crashes the compiler
+func rdar70764991() {
+  struct S {
+    static var foo: S { get { S() } }
+  }
+
+  func bar(_: Any, foo: String) {
+  }
+
+  func test(_ str: String) {
+    bar(str, foo: S.foo) // expected-error {{unnamed argument #1 must precede argument 'foo'}}  {{9-12=}} {{14-14=str, }}
+  }
+}