[astgen] Add begin and end source locs to BraceStmt visitor.

Author: zoecarver

lib/ASTGen/Sources/ASTGen/Stmts.swift

@@ -5,11 +5,12 @@ import SwiftSyntax
 extension ASTGenVisitor {
   public func visit(_ node: CodeBlockSyntax) -> ASTNode {
     let statements = node.statements.map(self.visit).map { $0.bridged() }
-    let loc = self.base.advanced(by: node.position.utf8Offset).raw
+    let startLoc = self.base.advanced(by: node.position.utf8Offset).raw
+    let endLoc = self.base.advanced(by: node.endPosition.utf8Offset).raw
 
     return .stmt(
       statements.withBridgedArrayRef { ref in
-        BraceStmt_create(ctx, loc, ref, loc)
+        BraceStmt_create(ctx, startLoc, ref, endLoc)
       })
   }