Fixed object lifetimes in ClangExpressionDeclMap

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