Only check newVal/newMethod privateWithin on -Xcheck-macros

Closes #17432

Author: nicolasstucki

compiler/src/scala/quoted/runtime/impl/QuotesImpl.scala

@@ -2540,11 +2540,11 @@ class QuotesImpl private (using val ctx: Context) extends Quotes, QuoteUnpickler
       def newMethod(owner: Symbol, name: String, tpe: TypeRepr): Symbol =
         newMethod(owner, name, tpe, Flags.EmptyFlags, noSymbol)
       def newMethod(owner: Symbol, name: String, tpe: TypeRepr, flags: Flags, privateWithin: Symbol): Symbol =
-        assert(!privateWithin.exists || privateWithin.isType, "privateWithin must be a type symbol or `Symbol.noSymbol`")
+        xCheckMacroAssert(!privateWithin.exists || privateWithin.isType, "privateWithin must be a type symbol or `Symbol.noSymbol`")
         checkValidFlags(flags.toTermFlags, Flags.validMethodFlags)
         dotc.core.Symbols.newSymbol(owner, name.toTermName, flags | dotc.core.Flags.Method, tpe, privateWithin)
       def newVal(owner: Symbol, name: String, tpe: TypeRepr, flags: Flags, privateWithin: Symbol): Symbol =
-        assert(!privateWithin.exists || privateWithin.isType, "privateWithin must be a type symbol or `Symbol.noSymbol`")
+        xCheckMacroAssert(!privateWithin.exists || privateWithin.isType, "privateWithin must be a type symbol or `Symbol.noSymbol`")
         checkValidFlags(flags.toTermFlags, Flags.validValFlags)
         dotc.core.Symbols.newSymbol(owner, name.toTermName, flags, tpe, privateWithin)
       def newBind(owner: Symbol, name: String, flags: Flags, tpe: TypeRepr): Symbol =