Rollup merge of #110694 - est31:builtin, r=petrochenkov

Implement builtin # syntax and use it for offset_of!(...)

Add `builtin #` syntax to the parser, as well as a generic infrastructure to support both item and expression position builtin syntaxes. The PR also uses this infrastructure for the implementation of the `offset_of!` macro, added by #106934.

cc `@petrochenkov` `@DrMeepster`

cc #110680 `builtin #` tracking issue
cc #106655 `offset_of!` tracking issue

compiler/rustc_span/src/symbol.rs

@@ -95,6 +95,7 @@ symbols! {
 
         // Weak keywords, have special meaning only in specific contexts.
         Auto:               "auto",
+        Builtin:            "builtin",
         Catch:              "catch",
         Default:            "default",
         MacroRules:         "macro_rules",
@@ -440,6 +441,7 @@ symbols! {
         breakpoint,
         bridge,
         bswap,
+        builtin_syntax,
         c_str,
         c_str_literals,
         c_unwind,