Added functionality to dematerialize values that were

used by the JIT compiled expression, including the
result of the expression.

Also added a new class, ASTType, which encapsulates an
opaque Clang type and its associated AST context.

Refactored ClangExpressionDeclMap to use ASTTypes,
significantly reducing the possibility of mixups of
types from different AST contexts.

llvm-svn: 108965