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Avoid UAF in WinCOFFObjectWriter with weak symbols.
When using weak symbols, the WinCOFFObjectWriter keeps a list (`WeakDefaults`) that's used to make names unique. This list should be reset when the object writer is reset, because otherwise reuse of the object writer can result in freed symbols being accessed. With some added output, this becomes clear when using `llc` in `--run-twice` mode: ``` $ ./llc --compile-twice -mtriple=x86_64-pc-win32 trivial.ll -filetype=obj DefineSymbol::WeakDefaults - .weak.foo.default - .weak.bar.default DefineSymbol::WeakDefaults - .weak.foo.default - áÑJij⌂ p§┼Ø┐☺ - .debug_macinfo.dw - .weak.bar.default ``` This does not seem to leak into the output object file though, so I couldn't come up with a test. I added one that just does `--run-twice` (and verified that it does access freed memory), which should result in detecting the invalid memory accesses when running under ASAN. Observed in a Julia PR where we started using weak symbols: JuliaLang/julia#45649 Differential Revision: https://reviews.llvm.org/D129840
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