[clang] [llvm] [Clang]: Enable speculative devirtualization (PR #159685)

[Teresa Johnson via llvm-commits]

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+// Test that the vtable metadata that are emitted by Clang when speculative devirtualization
+// is enabled can be used by the WholeProgramDevirt pass without being dropped on the way.
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teresajohnson wrote:

Since this is testing LLVM behavior, it should be an llvm side test - just take the LLVM IR generated from clang and move it under llvm/test/Transforms/DropUnnecessaryAssumes and use "opt" to test. You could test "opt -O3" as well as "opt -passes=drop-unnecessary-assumes".

https://github.com/llvm/llvm-project/pull/159685