Update CHANGELOG.md

CHANGELOG.md

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 ### Enhancements
 
 * hclwrite: New functions `TokensForTuple`, `TokensForObject`, and `TokensForFunctionCall` allow for more easily constructing the three constructs which are supported for static analysis and which HCL-based languages typically use in contexts where an expression is used only for its syntax, and not evaluated to produce a real value. For example, these new functions together are sufficient to construct all valid type constraint expressions from [the Type Expressions Extension](./ext/typeexpr/), which is the basis of variable type constraints in the Terraform language at the time of writing. ([#502](https://github.com/hashicorp/hcl/pull/502))
+* hclsyntax: The lexer will no longer allocate a small object on the heap for each token. Instead, in that situation it will allocate only when needed to return a diagnostic message with source location information. ([#490](https://github.com/hashicorp/hcl/pull/490))
 
 ## v2.11.1 (Dec 1, 2021)