[clang] [clang-repl] Refactor locking of runtime PTU stack (NFC) (PR #84176)

[Stefan Gränitz via cfe-commits]

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@@ -137,9 +136,10 @@ class Interpreter {
 
   Expr *SynthesizeExpr(Expr *E);
 
-private:
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weliveindetail wrote:

Ok, let me play with it a little more.

https://github.com/llvm/llvm-project/pull/84176