ASTGen: Resolve 'was never mutated' warnings.

Author: tshortli

lib/ASTGen/Sources/ASTGen/DeclAttrs.swift

@@ -936,7 +936,7 @@ extension ASTGenVisitor {
 
   func generateCustomAttr(attribute node: AttributeSyntax) -> BridgedCustomAttr? {
     guard
-      var args = node.arguments?.as(LabeledExprListSyntax.self)?[...]
+      let args = node.arguments?.as(LabeledExprListSyntax.self)?[...]
     else {
       // TODO: Diagnose.
       return nil

lib/ASTGen/Sources/ASTGen/Exprs.swift

@@ -317,7 +317,7 @@ extension ASTGenVisitor {
     additionalTrailingClosures: MultipleTrailingClosureElementListSyntax?
   ) -> BridgedArgumentList {
 
-    var bridgedArgs: BridgedArrayRef = {
+    let bridgedArgs: BridgedArrayRef = {
       // Arguments before ')'
       let normalArgs = labeledExprList.lazy.map({ elem in
         let labelInfo = elem.label.map(self.generateIdentifierAndSourceLoc(_:))
@@ -363,7 +363,7 @@ extension ASTGenVisitor {
     // of the normal arguments because we don't have a convenient way to pass
     // Optional to ASTBridging,  ASTBridging can know it's "nil" if
     // bridgedArgs.count == firstTrailingClosureIndex
-    var firstTrailingClosureIndex = labeledExprList.count
+    let firstTrailingClosureIndex = labeledExprList.count
 
     return BridgedArgumentList.createParsed(
       self.ctx,