Merge #1928

1928: Support `#[cfg(..)]` r=matklad a=oxalica

This PR implement `#[cfg(..)]` conditional compilation. It read default cfg options from `rustc --print cfg` with also hard-coded `test` and `debug_assertion` enabled.
Front-end settings are **not** included in this PR.

There is also a known issue that inner control attributes are totally ignored. I think it is **not** a part of `cfg` and create a separated issue for it. #1949

Fixes #1920 

Related: #1073 


Co-authored-by: uHOOCCOOHu <hooccooh1896@gmail.com>
Co-authored-by: oxalica <oxalicc@pm.me>

crates/ra_syntax/src/ast/generated.rs

@@ -1962,6 +1962,7 @@ impl AstNode for ModuleItem {
         }
     }
 }
+impl ast::AttrsOwner for ModuleItem {}
 impl ModuleItem {}
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub struct Name {

crates/ra_syntax/src/grammar.ron

@@ -397,7 +397,8 @@ Grammar(
         ),
         "ModuleItem": (
             enum: ["StructDef", "EnumDef", "FnDef", "TraitDef", "TypeAliasDef", "ImplBlock",
-                   "UseItem", "ExternCrateItem", "ConstDef", "StaticDef", "Module" ]
+                   "UseItem", "ExternCrateItem", "ConstDef", "StaticDef", "Module" ],
+            traits: ["AttrsOwner"]
         ),
         "ImplItem": (
             enum: ["FnDef", "TypeAliasDef", "ConstDef"]