We define a specialization of LLSDParam<const char*> to support passing an
LLSD object to a const char* function parameter. Needless to remark, passing
object.asString().c_str() would be Bad: destroying the temporary std::string
returned by asString() would immediately invalidate the pointer returned by
its c_str(). But when you pass LLSDParam<const char*>(object) as the
parameter, that specialization itself stores the std::string so the c_str()
pointer remains valid as long as the LLSDParam object does.

Then there's LLSDParam<LLSD>, used when we don't have the parameter type
available to select the LLSDParam specialization. LLSDParam<LLSD> defines a
templated conversion operator T() that constructs an LLSDParam<T> to provide
the actual parameter value. So far, so good.

The trouble was with the implementation of LLSDParam<LLSD>: it constructed a
_temporary_ LLSDParam<T>, implicitly called its operator T() and immediately
destroyed it. Destroying LLSDParam<const char*> destroyed its stored string,
thus invalidating the c_str() pointer before the target function was entered.

Instead, make LLSDParam<LLSD>::operator T() capture each LLSDParam<T> it
constructs, extending its lifespan to the lifespan of the LLSDParam<LLSD>
instance. For this, derive each LLSDParam specialization from LLSDParamBase, a
trivial base class that simply establishes the virtual destructor. We can then
capture any specialization as a pointer to LLSDParamBase.

Also restore LazyEventAPI tests on Mac.