[llvm] gn build: Add use_hwasan flag. (PR #143052)

[Peter Collingbourne via llvm-commits]

pcc wrote:

Some tests currently fail with this. Not sure why yet. They all seem to have something to do with dynamically loading with dlopen.
```
  LLVM :: BugPoint/attr-crash.ll
  LLVM :: BugPoint/compile-custom.ll
  LLVM :: BugPoint/crash-narrowfunctiontest.ll
  LLVM :: BugPoint/func-attrs-keyval.ll
  LLVM :: BugPoint/func-attrs.ll
  LLVM :: BugPoint/invalid-debuginfo.ll
  LLVM :: BugPoint/metadata.ll
  LLVM :: BugPoint/named-md.ll
  LLVM :: BugPoint/remove_arguments_test.ll
  LLVM :: BugPoint/replace-funcs-with-null.ll
  LLVM :: BugPoint/retain-crashing-metadata.ll
  LLVM :: BugPoint/unsymbolized.ll
  LLVM-Unit :: Passes/Plugins/./PluginsTests/0/2
  LLVM-Unit :: Passes/Plugins/./PluginsTests/1/2
```

https://github.com/llvm/llvm-project/pull/143052