Factor function-parameter utilities into a separate file

We are likely to grow more of these utilities, and they might
eventually be sunk down into a common library, so keep the factored
out.

Author: DougGregor

Sources/SwiftSyntaxMacros/CMakeLists.txt

@@ -21,7 +21,9 @@ add_swift_host_library(SwiftSyntaxMacros
 
   AbstractSourceLocation.swift
   BasicMacroExpansionContext.swift
+  FunctionParameterUtils.swift
   MacroExpansionContext.swift
+  MacroReplacement.swift
   MacroSystem.swift
   Syntax+MacroEvaluation.swift
 )

Sources/SwiftSyntaxMacros/FunctionParameterUtils.swift

@@ -0,0 +1,32 @@
+import SwiftSyntax
+
+extension FunctionParameterSyntax {
+  /// Retrieve the name of the parameter as it is used in source.
+  ///
+  /// Example:
+  ///
+  ///     func f(a: Int, _ b: Int, c see: Int) { ... }
+  ///
+  /// The parameter names for these three parameters are `a`, `b`, and `see`,
+  /// respectively.
+  var parameterName: TokenSyntax? {
+    // If there were two names, the second is the parameter name.
+    if let secondName = secondName {
+      if secondName.text == "_" {
+        return nil
+      }
+
+      return secondName
+    }
+
+    if let firstName = firstName {
+      if firstName.text == "_" {
+        return nil
+      }
+
+      return firstName
+    }
+
+    return nil
+  }
+}

Sources/SwiftSyntaxMacros/MacroReplacement.swift

@@ -2,37 +2,6 @@ import SwiftDiagnostics
 import SwiftSyntax
 import SwiftSyntaxBuilder
 
-extension FunctionParameterSyntax {
-  /// Retrieve the name of the parameter as it is used in source.
-  ///
-  /// Example:
-  ///
-  ///     func f(a: Int, _ b: Int, c see: Int) { ... }
-  ///
-  /// The parameter names for these three parameters are `a`, `b`, and `see`,
-  /// respectively.
-  var parameterName: TokenSyntax? {
-    // If there were two names, the second is the parameter name.
-    if let secondName = secondName {
-      if secondName.text == "_" {
-        return nil
-      }
-
-      return secondName
-    }
-
-    if let firstName = firstName {
-      if firstName.text == "_" {
-        return nil
-      }
-
-      return firstName
-    }
-
-    return nil
-  }
-}
-
 enum MacroExpanderError: DiagnosticMessage {
   case undefined
   case definitionNotMacroExpansion