Missing docs: don't require documenting Global Asm items.

global_asm! items cannot be documented, the lint still gets triggered
after adding documentation to the macro invocation. Furthermore, even
if we could add documentation to the AST node, rustdoc doesn't render
it anyways.

Playground example: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=5182df182f0ffbbab4c3107e43368ac3

clippy_lints/src/missing_doc.rs

@@ -133,12 +133,12 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MissingDoc {
             hir::ItemKind::Struct(..) => "a struct",
             hir::ItemKind::Trait(..) => "a trait",
             hir::ItemKind::TraitAlias(..) => "a trait alias",
-            hir::ItemKind::GlobalAsm(..) => "an assembly blob",
             hir::ItemKind::Ty(..) => "a type alias",
             hir::ItemKind::Union(..) => "a union",
             hir::ItemKind::Existential(..) => "an existential type",
             hir::ItemKind::ExternCrate(..)
             | hir::ItemKind::ForeignMod(..)
+            | hir::ItemKind::GlobalAsm(..)
             | hir::ItemKind::Impl(..)
             | hir::ItemKind::Use(..) => return,
         };

tests/ui/missing-doc.rs

@@ -2,7 +2,7 @@
 // When denying at the crate level, be sure to not get random warnings from the
 // injected intrinsics by the compiler.
 #![allow(dead_code)]
-#![feature(associated_type_defaults)]
+#![feature(associated_type_defaults, global_asm)]
 
 //! Some garbage docs for the crate here
 #![doc = "More garbage"]
@@ -176,3 +176,6 @@ pub mod public_interface {
 }
 
 fn main() {}
+
+// Ensure global asm doesn't require documentation.
+global_asm! { "" }