Bug #:

Submitted by:
Reviewed by:
Fixed a bug in Sema::CheckAddressOfOperand(). It was (incorrectly) using
isModifiableLvalue() instead of isLvalue(). This motivated me to (finally)
cleanup methods surrounding lsLvalue/isModifiableLvalue. Cleanup involved:
- adding Expr::isLvalue().
- modified Expr::isModifiableLvalue() to use Expr::isLvalue().
- removed Type::isLvalue(), Type::isModifiableLvalue(), and
QualType::isModifiableLvalue(). They were confusing...the respective logic
is now a part of the Expr member functions...
- also added some comments and spec references, since these methods are
so central to expressions working properly.

llvm-svn: 39443