Make CompilerBuildConfiguration a struct again

Author: DougGregor

lib/ASTGen/Sources/ASTGen/ASTGen+CompilerBuildConfiguration.swift

@@ -25,6 +25,7 @@ extension ASTGenVisitor {
   /// produced due to the evaluation.
   func activeClause(in node: IfConfigDeclSyntax) -> IfConfigClauseSyntax? {
     // Determine the active clause.
+    var buildConfiguration = self.buildConfiguration
     buildConfiguration.conditionLoc = generateSourceLoc(node)
     let (activeClause, diagnostics) = node.activeClause(in: buildConfiguration)
     diagnoseAll(diagnostics)

lib/ASTGen/Sources/ASTGen/CompilerBuildConfiguration.swift

@@ -18,7 +18,7 @@ import SwiftSyntax
 
 /// A build configuration that uses the compiler's ASTContext to answer
 /// queries.
-final class CompilerBuildConfiguration: BuildConfiguration {
+struct CompilerBuildConfiguration: BuildConfiguration {
   let ctx: BridgedASTContext
   var conditionLoc: BridgedSourceLoc