Fix up llexception.h's cross-platform SEH wrapper.

Introduce AlwaysReturn<void> specialization, which always discards any result
of calling the specified callable with specified args.

Derive new Windows_SEH_exception from LLException, not std::runtime_error.

Put the various SEH functions in LL::seh nested namespace, e.g.
LL::seh::catcher() as the primary API.

Break out more levels of Windows SEH handler to work around the restrictions on
functions containing __try/__except.

The triadic catcher() overload now does little save declare a std::string
stacktrace before forwarding the call to catcher_inner(), passing a reference
to stacktrace along with the trycode, filter and handler functions.

catcher_inner() accepts the stacktrace and the three function template
arguments. It contains the __try/__except logic. It calls a new filter_()
wrapper template, which calls fill_stacktrace() before forwarding the call to
the caller's filter function. fill_stacktrace(), in the .cpp file, contains
the logic to populate the stacktrace string -- unless the Structured Exception
is stack overflow, in which case it puts an explanatory string instead.

catcher_inner()'s __except clause passes not only the code, but also the
stacktrace string, to the caller's handler function. It wraps the caller's
handler function in always_return<rtype>(), where rtype is the type returned
by the trycode function. This allows a handler to return a value, while also
supporting the void handler case, e.g. one that throws a C++ exception. (This
is why we need AlwaysReturn<void>: some trycode() functions are themselves
void.)

For the dyadic catcher() overload, introduce common_filter() containing the
logic to distinguish a C++ exception from any other kind of Structured
Exception. The fact that the stacktrace is captured before the filter function
is called should permit capturing a stacktrace for a C++ exception as well as
for most other Structured Exceptions.

As before, the monadic catcher() overload supplies the rethrow() handler, in
the .cpp file.

Change existing calls from seh_catcher() to LL::seh::catcher().

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