Rollup merge of #47496 - QuietMisdreavus:rls-doc-include, r=estebank

add documentation from doc(include) to analysis data

cc #44732

Currently save-analysis only loads docs from plain doc comments and doc attributes. Since `#[doc(include="filename.md")]` doesn't create a plain doc attribute when it loads the file, we need to be sure to pick up this info for the analysis data.

src/test/run-make/save-analysis/extra-docs.md

@@ -0,0 +1 @@
+Extra docs for this struct.

src/test/run-make/save-analysis/foo.rs

@@ -12,6 +12,7 @@
 #![feature(box_syntax)]
 #![feature(rustc_private)]
 #![feature(associated_type_defaults)]
+#![feature(external_doc)]
 
 extern crate graphviz;
 // A simple rust project
@@ -461,3 +462,6 @@ impl Iterator for SilenceGenerator {
 trait Foo {
     type Bar = FrameBuffer;
 }
+
+#[doc(include="extra-docs.md")]
+struct StructWithDocs;