implicit lvalue-to-rvalue casts that John McCall
recently introduced.  This causes a whole bunch
of logic in the analyzer for handling lvalues
to vanish.  It does, however, raise a few issues
in the analyzer w.r.t to modeling various constructs
(e.g., field accesses to compound literals).

The .c/.m analysis test cases that fail are
due to a missing lvalue-to-rvalue cast that
will get introduced into the AST.  The .cpp
failures were more than I could investigate in
one go, and the patch was already getting huge.
I have XFAILED some of these tests, and they
should obviously be further investigated.

Some highlights of this patch include:

- CFG no longer requires an lvalue bit for
  CFGElements
- StackFrameContext doesn't need an 'asLValue'
  flag
- The "VisitLValue" path from GRExprEngine has
  been eliminated.

Besides the test case failures (XFAILed), there
are surely other bugs that are fallout from
this change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@121960 91177308-0d34-0410-b5e6-96231b3b80d8