Fix wrong suggestions for T:

This commit fixes a corner case in `suggest_constraining_type_params`
that was causing incorrect suggestions.

For the following functions:
```rust
fn a<T:>(t: T) { [t, t]; }
fn b<T>(t: T) where T: { [t, t]; }
```
We previously suggested the following:
```text
...
help: consider restricting type parameter `T`
  |
1 | fn a<T: Copy:>(t: T) { [t, t]; }
  |       ++++++
...
help: consider further restricting this bound
  |
2 | fn b<T>(t: T) where T: + Copy { [t, t]; }
  |                        ++++++
```
Note that neither `T: Copy:` not `where T: + Copy` is a correct bound.

With this commit the suggestions are correct:
```text
...
help: consider restricting type parameter `T`
  |
1 | fn a<T: Copy>(t: T) { [t, t]; }
  |         ++++
...
help: consider further restricting this bound
  |
2 | fn b<T>(t: T) where T: Copy { [t, t]; }
  |                        ++++
```

compiler/rustc_hir/src/hir.rs

@@ -17,7 +17,7 @@ use rustc_error_messages::MultiSpan;
 use rustc_index::vec::IndexVec;
 use rustc_macros::HashStable_Generic;
 use rustc_span::hygiene::MacroKind;
-use rustc_span::source_map::Spanned;
+use rustc_span::source_map::{SourceMap, Spanned};
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use rustc_span::{def_id::LocalDefId, BytePos, Span, DUMMY_SP};
 use rustc_target::asm::InlineAsmRegOrRegClass;
@@ -523,6 +523,40 @@ impl<'hir> GenericParam<'hir> {
             })
             .map(|sp| sp.shrink_to_hi())
     }
+
+    /// Returns the span of `:` after a generic parameter.
+    ///
+    /// For example:
+    ///
+    /// ```text
+    /// fn a<T:>()
+    ///       ^
+    ///       |      here
+    ///       here   |
+    ///              v
+    /// fn b<T       :>()
+    ///
+    /// fn c<T
+    ///
+    /// :>()
+    /// ^
+    /// |
+    /// here
+    /// ```
+    pub fn colon_span_for_suggestions(&self, source_map: &SourceMap) -> Option<Span> {
+        let sp = source_map
+            .span_extend_while(self.span.shrink_to_hi(), |c| c.is_whitespace() || c == ':')
+            .ok()?;
+
+        let snippet = source_map.span_to_snippet(sp).ok()?;
+        let offset = snippet.find(':')?;
+
+        let colon_sp = sp
+            .with_lo(BytePos(sp.lo().0 + offset as u32))
+            .with_hi(BytePos(sp.lo().0 + (offset + ':'.len_utf8()) as u32));
+
+        Some(colon_sp)
+    }
 }
 
 #[derive(Default)]