Make check flags for newMethod, newVal and newBind in Quotes API less restrictive (#18217)

Fixes #17764

Allows `Flags.Artifact` and `Flags.Synthetic` and the other ["flags that
could be
allowed"](https://github.com/lampepfl/dotty/blob/3.3.1-RC1/compiler/src/scala/quoted/runtime/impl/QuotesImpl.scala#L2880-L2885)
as they are valid flags and there is no reason to forbid them.

Author: nicolasstucki

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

@@ -2915,9 +2915,10 @@ class QuotesImpl private (using val ctx: Context) extends Quotes, QuoteUnpickler
       def Transparent: Flags = dotc.core.Flags.Transparent
 
       // Keep: aligned with Quotes's `newMethod` doc
-      private[QuotesImpl] def validMethodFlags: Flags = Private | Protected | Override | Deferred | Final | Method | Implicit | Given | Local | AbsOverride | JavaStatic // Flags that could be allowed: Synthetic | ExtensionMethod | Exported | Erased | Infix | Invisible
+      private[QuotesImpl] def validMethodFlags: Flags = Private | Protected | Override | Deferred | Final | Method | Implicit | Given | Local | AbsOverride | JavaStatic | Synthetic | Artifact // Flags that could be allowed: Synthetic | ExtensionMethod | Exported | Erased | Infix | Invisible
       // Keep: aligned with Quotes's `newVal` doc
-      private[QuotesImpl] def validValFlags: Flags = Private | Protected | Override | Deferred | Final | Param | Implicit | Lazy | Mutable | Local | ParamAccessor | Module | Package | Case | CaseAccessor | Given | Enum | AbsOverride | JavaStatic // Flags that could be added: Synthetic | Erased | Invisible
+      private[QuotesImpl] def validValFlags: Flags = Private | Protected | Override | Deferred | Final | Param | Implicit | Lazy | Mutable | Local | ParamAccessor | Module | Package | Case | CaseAccessor | Given | Enum | AbsOverride | JavaStatic | Synthetic | Artifact // Flags that could be added: Synthetic | Erased | Invisible
+
       // Keep: aligned with Quotes's `newBind` doc
       private[QuotesImpl] def validBindFlags: Flags = Case // Flags that could be allowed: Implicit | Given | Erased
     end Flags

library/src/scala/quoted/Quotes.scala

@@ -3816,7 +3816,7 @@ trait Quotes { self: runtime.QuoteUnpickler & runtime.QuoteMatching =>
       *  @param parent The owner of the method
       *  @param name The name of the method
       *  @param tpe The type of the method (MethodType, PolyType, ByNameType)
-      *  @param flags extra flags to with which the symbol should be constructed. `Method` flag will be added. Can be `Private | Protected | Override | Deferred | Final | Method | Implicit | Given | Local | JavaStatic`
+      *  @param flags extra flags to with which the symbol should be constructed. `Method` flag will be added. Can be `Private | Protected | Override | Deferred | Final | Method | Implicit | Given | Local | JavaStatic | Synthetic | Artifact`
       *  @param privateWithin the symbol within which this new method symbol should be private. May be noSymbol.
       */
       // Keep: `flags` doc aligned with QuotesImpl's `validMethodFlags`
@@ -3833,7 +3833,7 @@ trait Quotes { self: runtime.QuoteUnpickler & runtime.QuoteMatching =>
       *  @param parent The owner of the val/var/lazy val
       *  @param name The name of the val/var/lazy val
       *  @param tpe The type of the val/var/lazy val
-      *  @param flags extra flags to with which the symbol should be constructed. Can be `Private | Protected | Override | Deferred | Final | Param | Implicit | Lazy | Mutable | Local | ParamAccessor | Module | Package | Case | CaseAccessor | Given | Enum | JavaStatic`
+      *  @param flags extra flags to with which the symbol should be constructed. Can be `Private | Protected | Override | Deferred | Final | Param | Implicit | Lazy | Mutable | Local | ParamAccessor | Module | Package | Case | CaseAccessor | Given | Enum | JavaStatic | Synthetic | Artifact`
       *  @param privateWithin the symbol within which this new method symbol should be private. May be noSymbol.
       *  @note As a macro can only splice code into the point at which it is expanded, all generated symbols must be
       *        direct or indirect children of the reflection context's owner.