Force a load when creating a reference to a temporary copied from a bitfield.

For this source:
  const int &ref = someStruct.bitfield;

We used to generate this AST:

  DeclStmt [...]
  `-VarDecl [...] ref 'const int &'
    `-MaterializeTemporaryExpr [...] 'const int' lvalue
      `-ImplicitCastExpr [...] 'const int' lvalue <NoOp>
        `-MemberExpr [...] 'int' lvalue bitfield .bitfield [...]
          `-DeclRefExpr [...] 'struct X' lvalue ParmVar [...] 'someStruct' 'struct X'

Notice the lvalue inside the MaterializeTemporaryExpr, which is very
confusing (and caused an assertion to fire in the analyzer - PR15694).

We now generate this:

  DeclStmt [...]
  `-VarDecl [...] ref 'const int &'
    `-MaterializeTemporaryExpr [...] 'const int' lvalue
      `-ImplicitCastExpr [...] 'int' <LValueToRValue>
        `-MemberExpr [...] 'int' lvalue bitfield .bitfield [...]
          `-DeclRefExpr [...] 'struct X' lvalue ParmVar [...] 'someStruct' 'struct X'

Which makes a lot more sense. This allows us to remove code in both
CodeGen and AST that hacked around this special case.

The commit also makes Clang accept this (legal) C++11 code:

  int &&ref = std::move(someStruct).bitfield

PR15694 / <rdar://problem/13600396>