Suggest path separator for single-colon typos

This commit adds guidance for when a user means to type a path, but ends
up typing a single colon, such as `<<Impl as T>:Ty>`.

This change seemed pertinent as the current error message is
particularly misleading, emitting `error: unmatched angle bracket`,
despite the angle bracket being matched later on, leaving the user to
track down the typo'd colon.

src/test/ui/parser/qualified-path-in-turbofish.fixed

@@ -0,0 +1,19 @@
+// run-rustfix
+trait T {
+    type Ty;
+}
+
+struct Impl;
+
+impl T for Impl {
+    type Ty = u32;
+}
+
+fn template<T>() -> i64 {
+    3
+}
+
+fn main() {
+    template::<<Impl as T>::Ty>();
+    //~^ ERROR found single colon where type path was expected
+}

src/test/ui/parser/qualified-path-in-turbofish.rs

@@ -0,0 +1,19 @@
+// run-rustfix
+trait T {
+    type Ty;
+}
+
+struct Impl;
+
+impl T for Impl {
+    type Ty = u32;
+}
+
+fn template<T>() -> i64 {
+    3
+}
+
+fn main() {
+    template::<<Impl as T>:Ty>();
+    //~^ ERROR found single colon where type path was expected
+}

src/test/ui/parser/qualified-path-in-turbofish.stderr

@@ -0,0 +1,8 @@
+error: found single colon where type path was expected
+  --> $DIR/qualified-path-in-turbofish.rs:17:27
+   |
+LL |     template::<<Impl as T>:Ty>();
+   |                           ^ help: use double colon: `::`
+
+error: aborting due to previous error
+