[clang] [clang-repl] Suppress [[nodiscard]] warnings for REPL printed express (PR #178661)

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bala-bhargav wrote:

@vgvassilev
 I've updated the PR to address all your comments — the fix is now entirely confined to 
Interpreter.cpp with no changes to ParseDecl.cpp or any other file.  The approach is i  suppresses warn_unused_result before parsing (since we don't know the semicolon status at that point), and after parsing, re-enables the warning and explicitly diagnoses [[nodiscard]] only for 
TopLevelStmtDecl
 expressions where the semicolon is present (i.e., the value is genuinely being discarded). A test has been added covering both scenarios. I would appreciate your review, and please do let me know if you'd prefer a different approach — I'm happy to follow in your guidance.

https://github.com/llvm/llvm-project/pull/178661