[MC] 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

Reviewed By: mstorsjo

Differential Revision: https://reviews.llvm.org/D129840

Author: maleadt

llvm/lib/MC/WinCOFFObjectWriter.cpp

@@ -169,6 +169,7 @@ class WinCOFFObjectWriter : public MCObjectWriter {
     Strings.clear();
     SectionMap.clear();
     SymbolMap.clear();
+    WeakDefaults.clear();
     MCObjectWriter::reset();
   }
 

llvm/test/MC/COFF/weak-uaf.ll

@@ -0,0 +1,12 @@
+; RUN: llc --compile-twice -mtriple=x86_64-pc-win32 -filetype=obj < %s
+
+; UAF when re-using the MCObjectWriter. does not leak into the output,
+; but should be detectable with --compile-twice under ASAN or so.
+
+define weak void @foo() nounwind {
+  ret void
+}
+
+define weak void @bar() nounwind {
+  ret void
+}