Flipped the big switch: LLDB now uses the new
Materializer for all expressions that need to
run in the target.  This includes the following
changes:

- Removed a bunch of (de-)materialization code
  from ClangExpressionDeclMap and assumed the
  presence of a Materializer where we previously
  had a fallback.

- Ensured that an IRMemoryMap is passed into
  ClangExpressionDeclMap::Materialize().

- Fixed object ownership on LLVMContext; it is
  now owned by the IRExecutionUnit, since the
  Module and the ExecutionEngine both depend on
  its existence.

- Fixed a few bugs in IRMemoryMap and the
  Materializer that showed up during testing.