In preparation for factoring persistent variables into a generic part and a

Clang-specific part, create the ExpressionVariable source/header file and
move ClangExpressionVariable into the Clang expression parser plugin.

It is expected that there are some ugly #include paths... these will be resolved
by either (1) making that code use generic expression variables (once they're
separated appropriately) or (2) moving that code into a plug-in, often
the expression parser plug-in.

llvm-svn: 246737