Revert "Reverted a recent change that appears to have affected performance on some projects. This attempts to address #10832. (#10841)"

This reverts commit fe7aaf2.

Author: erictraut

packages/pyright-internal/src/parser/tokenizer.ts

@@ -1616,7 +1616,7 @@ export class Tokenizer {
         let isInNamedUnicodeEscape = false;
         const start = this._cs.position;
         let escapedValueLength = 0;
-        const getEscapedValue = () => this._cs.getText().slice(start, start + escapedValueLength);
+        const getEscapedValue = () => cloneStr(this._cs.getText().slice(start, start + escapedValueLength));
 
         while (true) {
             if (this._cs.isEndOfStream()) {