[clang] Fix crash and improve diagnostics for void lambda returns (PR #188904)

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``````````bash
git-clang-format --diff origin/main HEAD --extensions cpp -- clang/test/SemaCXX/void-lambda-return-init.cpp clang/lib/Sema/SemaStmt.cpp clang/test/CXX/expr/expr.prim/expr.prim.lambda/p4.cpp --diff_from_common_commit
``````````

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View the diff from clang-format here.
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``````````diff
diff --git a/clang/lib/Sema/SemaStmt.cpp b/clang/lib/Sema/SemaStmt.cpp
index 5daad9250..4a906cbc7 100644
--- a/clang/lib/Sema/SemaStmt.cpp
+++ b/clang/lib/Sema/SemaStmt.cpp
@@ -3676,7 +3676,7 @@ StmtResult Sema::ActOnCapScopeReturnStmt(SourceLocation ReturnLoc,
         // though we represent it as one). We still deduce 'void'.
         Diag(ReturnLoc, diag::err_lambda_return_init_list)
           << RetValExp->getSourceRange();
-          RetValExp = nullptr; 
+        RetValExp = nullptr;
       }
 
       FnRetType = Context.VoidTy;

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https://github.com/llvm/llvm-project/pull/188904