Rename Expr::Evaluate to Expr::EvaluateAsRValue to make it clear that it will

Rename Expr::Evaluate to Expr::EvaluateAsRValue to make it clear that it will
implicitly perform an lvalue-to-rvalue conversion if used on an lvalue
expression. Also improve the documentation of Expr::Evaluate* to indicate which
of them will accept expressions with side-effects.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@143263 91177308-0d34-0410-b5e6-96231b3b80d8