Eliminated redundant "constructors" for ClangExpressionVariable. ...

Eliminated redundant "constructors" for ClangExpressionVariable.                                                                                                                                                       
                                                                                                                                                                                                                       
The ClangExpressionVariable::CreateVariableInList functions looked cute, but                                                                                                                                           
caused more confusion than they solved.  I removed them, and instead made sure                                                                                                                                         
that there are adequate facilities for easily adding newly-constructed                                                                                                                                                 
ExpressionVariables to lists.      

I also made some of the constructors that are common be generic, so that it's
possible to construct expression variables from generic places (like the ABI and
ValueObject) without having to know the specifics about the class.

llvm-svn: 249095