[RFR]: Add __tsan_symbolize_external_ex

[Man Cao]

Looks good.
Minor comment is that you could add some comment on
how __tsan_symbolize_external_ex() differs from other functions starting
with "__tsan", and why it does not need to be inside tsanExternalDecls.hpp.
No need for a new webrev.

-Man


On Wed, Mar 27, 2019 at 8:57 AM Arthur Eubanks <aeubanks at google.com> wrote:

> >
> > Any reason you want to do it two steps?
> >
> This slightly differs from our internal patch where we have a wrapper in
> the launcher. I wanted to see if this difference made sense.
>