Rollup merge of rust-lang#42723 - ubsan:master, r=QuietMisdreavus

Add `_` to the list of keywords

also, make sure the keyword table is correctly spaced

note: the reference also needs to be updated. This is not the only way to do this in the grammar, but it's my preferred way.

src/doc/grammar.md

@@ -154,19 +154,19 @@ token : simple_token | ident | literal | symbol | whitespace token ;
 
 <p id="keyword-table-marker"></p>
 
-| | | | | |
-|----------|----------|----------|----------|---------|
-| abstract | alignof | as  | become | box  |
-| break | const | continue | crate | do  |
-| else | enum | extern | false | final |
-| fn | for | if | impl | in  |
-| let | loop | macro | match | mod  |
-| move | mut | offsetof | override | priv  |
-| proc | pub | pure | ref  | return |
-| Self | self | sizeof | static  | struct |
-| super | trait | true | type  | typeof |
-| unsafe | unsized | use | virtual | where  |
-| while | yield |  | | |
+| | | | |  |
+|----------|----------|----------|----------|----------|
+| _  | abstract | alignof | as  | become |
+| box  | break  | const | continue | crate |
+| do  | else | enum  | extern | false  |
+| final | fn | for | if | impl |
+| in  | let  | loop  | macro | match |
+| mod | move | mut | offsetof | override |
+| priv | proc | pub | pure | ref |
+| return | Self | self | sizeof | static |
+| struct | super | trait | true | type |
+| typeof | unsafe | unsized | use | virtual |
+| where | while | yield | |  |
 
 
 Each of these keywords has special meaning in its grammar, and all of them are