gn build: Add use_hwasan flag.

[pcc]

No description provided.

[pcc]

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

[pcc]

Looks like it's because of #57206, I guess we could set LLVM_ENABLE_PLUGINS based on use_hwasan for now.