Introduce Expr::Classify and Expr::ClassifyModifiable, which determine the...

Introduce Expr::Classify and Expr::ClassifyModifiable, which determine the classification of an expression under the C++0x taxology (value category). Reimplement isLvalue and isModifiableLvalue using these functions. No regressions in the test suite from this, and my rough performance check doesn't show any regressions either.

llvm-svn: 107007