This adds Rust support to Mangled.  I am not completely certain that
this is needed (or alternatively that it does enough, maybe
Mangled::GuessLanguage needs a Rust case).  This should be checked
before attempting to upstream.

This was needed for the Rust plugin

Add a TypeAndOrName constructor that was declared but not defined.
This is used in the Rust plugin.  See https://reviews.llvm.org/D44752

Introduce LLDB_PY_LIB_SUFFIX and use it in various places in the
build.  This lets the x.py-based build work properly without having to
set LLVM_LIBDIR_SUFFIX.

See https://bugs.llvm.org/show_bug.cgi?id=18957 for some discussion.

Sometimes the DWARF can omit information about a discriminant, for
example when an Option shares a discriminant slot with an enum that it
wraps.  In this case, lldb could crash, because the discriminant was
not found and because there was no default variant.

No test case because this relies on a compiler bug that will soon be
fixed.

Fixes #16