KT-16264 Forbid usage of _ without backticks

Forbid underscore-only (_, __, ___, ...) names as callees and as types.

If CHECK_TYPE directive is on, filter out UNDERSCORE_USAGE_WITHOUT_BACKTICKS messages.

compiler/testData/diagnostics/ReadMe.md

@@ -58,6 +58,8 @@ With that, an exact type of an expression can be checked in the following way:
        expr checkType { _<A>() }
     }
 
+`CHECK_TYPE` directive also disables `UNDERSCORE_USAGE_WITHOUT_BACKTICKS` diagnostics output.
+
 #### Usage:
 
     // !CHECK_TYPE