Make the MacroEvaluationContext initializer public

Author: DougGregor

Sources/_SwiftSyntaxMacros/MacroEvaluationContext.swift

@@ -21,7 +21,6 @@ public struct MacroEvaluationContext {
   /// Used to map the provided syntax nodes into source locations.
   public let sourceLocationConverter: SourceLocationConverter
 
-  @_spi(Testing)
   public init(
     moduleName: String,
     sourceLocationConverter: SourceLocationConverter