so that it is not referring to potentially stale
state during IR execution.

This was done by introducing modular state (like
ClangExpressionVariable) where groups of state
variables have well-defined lifetimes:

- m_parser_vars are specific to parsing, and only
  exist between calls to WillParse() and DidParse().

- m_struct_vars survive for the entire execution
  of the ClangExpressionDeclMap because they
  provide the template for a materialized set of
  expression variables.

- m_material_vars are specific to a single
  instance of materialization, and only exist
  between calls to Materialize() and
  Dematerialize().

I also removed unnecessary references to long-
lived state that really didn't need to be referred
to at all, and also introduced several assert()s
that helped me diagnose a few bugs (fixed too).

llvm-svn: 120778