Changed the expression parser to return the lvalue flag as part of the

Changed the expression parser to return the lvalue flag as part of the
ExprDesc structure, not as separate value.
WARNING: The current code does compile but does not work correctly, because
the lvalue flag is part of ExprDesc.Flags and not masked out in several tests
throughout the code.

git-svn-id: svn://svn.cc65.org/cc65/trunk@3046 b7a2c559-68d2-44c3-8de9-860c34a00d81