[CSDiagnostics] Ensure fix-it inserts uses correct location in infered key path root optional unwrapped

Author: LucianoPAlmeida

lib/Sema/CSDiagnostics.cpp

@@ -1042,6 +1042,8 @@ SourceRange MemberAccessOnOptionalBaseFailure::getSourceRange() const {
     if (componentPathElt->getIndex() == 0) {
       if (auto rootType = keyPathExpr->getRootType()) {
         return rootType->getSourceRange();
+      } else {
+        return keyPathExpr->getComponents().front().getLoc();
       }
     } else {
       auto componentIdx = componentPathElt->getIndex() - 1;

test/expr/unary/keypath/keypath.swift

@@ -1012,6 +1012,11 @@ func testMemberAccessOnOptionalKeyPathComponent() {
   let _ : KeyPath<String?, Int> = \.count // expected-error {{key path root inferred as optional type 'String?' must be unwrapped to refer to member 'count' of unwrapped type 'String'}}
   // expected-note@-1 {{chain the optional using '?.' to access unwrapped type member 'count'}} {{37-37=?.}}
   // expected-note@-2 {{unwrap the optional using '!.' to access unwrapped type member 'count'}} {{37-37=!.}}
+
+  let _ : KeyPath<String?, Int> = \.utf8.count 
+  // expected-error@-1 {{key path root inferred as optional type 'String?' must be unwrapped to refer to member 'utf8' of unwrapped type 'String'}}
+  // expected-note@-2 {{chain the optional using '?.' to access unwrapped type member 'utf8'}} {{37-37=?.}}
+  // expected-note@-3 {{unwrap the optional using '!.' to access unwrapped type member 'utf8'}} {{37-37=!.}}
 }
 
 func testSyntaxErrors() {