[xray] Apply default visibility to __xray_CustomEvent/__xray_TypedEvent

`__xray_customevent` and `__xray_typedevent` are built-in functions in
Clang. With -fxray-instrument, they are lowered to `__xray_CustomEvent`
(with 2 arguments) or `__xray_TypedEvent` (with 3 arguments).

xray patching is supported for shared objects, but they may contain
`__xray_customevent` and `__xray_typedevent` references that need to be
satisfied by default visibility definitions exported by the executable.

lld since df54f627, like GNU ld, catches
the scenario at link time.