Introduce -Zprofile-closures to evaluate the impact of 2229

This creates a CSV with name "closure_profile_XXXXX.csv", where the
variable part is the process id of the compiler.

To profile a cargo project you can run one of the following depending on
if you're compiling a library or a binary:

```
cargo +stage1 rustc --lib -- -Zprofile-closures
cargo +stage1 rustc --bin -- -Zprofile-closures
```

compiler/rustc_middle/src/ty/mod.rs

@@ -173,6 +173,25 @@ pub enum Visibility {
     Invisible,
 }
 
+#[derive(
+    Clone,
+    Debug,
+    PartialEq,
+    Eq,
+    Copy,
+    Hash,
+    TyEncodable,
+    TyDecodable,
+    HashStable,
+    TypeFoldable
+)]
+pub struct ClosureSizeProfileData<'tcx> {
+    /// Tuple containing the types of closure captures before the feature `capture_disjoint_fields`
+    pub before_feature_tys: Ty<'tcx>,
+    /// Tuple containing the types of closure captures after the feature `capture_disjoint_fields`
+    pub after_feature_tys: Ty<'tcx>,
+}
+
 pub trait DefIdTree: Copy {
     fn parent(self, id: DefId) -> Option<DefId>;