[Diagnostics] Add a fix-it for misplaced throws in function types (#25306)

Adds a diagnostic error for misplaced throws in function types. For e.g:

```swift
let fn: () -> throws Void  // expected-error{{'throws' may only occur before '->'}} {{12-12=throws }} {{15-22=}}
```

Resolves SR-10703.

Author: tapthaker

lib/Parse/ParseType.cpp

@@ -417,8 +417,15 @@ ParserResult<TypeRepr> Parser::parseType(Diag<> MessageID,
   if (Tok.is(tok::arrow)) {
     // Handle type-function if we have an arrow.
     SourceLoc arrowLoc = consumeToken();
+    if (Tok.is(tok::kw_throws)) {
+      Diag<> DiagID = diag::throws_in_wrong_position;
+      diagnose(Tok.getLoc(), DiagID)
+          .fixItInsert(arrowLoc, "throws ")
+          .fixItRemove(Tok.getLoc());
+      throwsLoc = consumeToken();
+    }
     ParserResult<TypeRepr> SecondHalf =
-      parseType(diag::expected_type_function_result);
+        parseType(diag::expected_type_function_result);
     if (SecondHalf.hasCodeCompletion())
       return makeParserCodeCompletionResult<TypeRepr>();
     if (SecondHalf.isNull())

test/Parse/errors.swift

@@ -125,3 +125,5 @@ func fixitThrow2() throws {
   throw MSV.Foo
   var _: (Int) throw -> Int // expected-error{{expected throwing specifier; did you mean 'throws'?}} {{16-21=throws}}
 }
+
+let fn: () -> throws Void  // expected-error{{'throws' may only occur before '->'}} {{12-12=throws }} {{15-22=}}