Add with_{def_site,call_site,legacy}_ctxt, methods to Span

Use these to create call-site spans for AST passes when needed.

src/libsyntax_pos/lib.rs

@@ -514,6 +514,25 @@ impl Span {
                   span.ctxt)
     }
 
+    /// Equivalent of `Span::def_site` from the proc macro API,
+    /// except that the location is taken from the `self` span.
+    pub fn with_def_site_ctxt(self, expn_id: ExpnId) -> Span {
+        self.with_ctxt_from_mark(expn_id, Transparency::Opaque)
+    }
+
+    /// Equivalent of `Span::call_site` from the proc macro API,
+    /// except that the location is taken from the `self` span.
+    pub fn with_call_site_ctxt(&self, expn_id: ExpnId) -> Span {
+        self.with_ctxt_from_mark(expn_id, Transparency::Transparent)
+    }
+
+    /// Span with a context reproducing `macro_rules` hygiene (hygienic locals, unhygienic items).
+    /// FIXME: This should be eventually replaced either with `with_def_site_ctxt` (preferably),
+    /// or with `with_call_site_ctxt` (where necessary).
+    pub fn with_legacy_ctxt(&self, expn_id: ExpnId) -> Span {
+        self.with_ctxt_from_mark(expn_id, Transparency::SemiTransparent)
+    }
+
     /// Produces a span with the same location as `self` and context produced by a macro with the
     /// given ID and transparency, assuming that macro was defined directly and not produced by
     /// some other macro (which is the case for built-in and procedural macros).