[FIR] Fix the TODO in FirConflictsHelpers

According to
https://github.com/Kotlin/KEEP/blob/master/proposals/enhancing-main-convention.md#rules-and-semantics,
only main function without args may avoid
`CONFLICTING_OVERLOADS`, but since the
jps/jps-plugin/testData/incremental/withJava/other/mainRedeclaration
test dates back to 2015, this is not the
current behavior.

core/compiler.common/src/org/jetbrains/kotlin/builtins/StandardNames.kt

@@ -36,6 +36,8 @@ object StandardNames {
 
     @JvmField val NAME = Name.identifier("name")
 
+    @JvmField val MAIN = Name.identifier("main")
+
     @JvmField val NEXT_CHAR = Name.identifier("nextChar")
 
     @JvmField val IMPLICIT_LAMBDA_PARAMETER_NAME = Name.identifier("it")