Patch to separate lvalue_or_else for C and C++

[Mark Mitchell]

Joseph S. Myers wrote:
> Stopping the C front end from depending on NON_LVALUE_EXPR requires
> that it determine information about whether an expression is an lvalue
> from internal structures (additional flags in c_expr) rather than from
> the tree for the expression.  Thus lvalue_or_else needs to become
> local to each of the C and C++ front ends again, although the code
> printing diagnostics can remain shared.  This patch makes it local to
> the front ends, with a shared lvalue_error function to deal with the
> diagnostics.
> 
> Bootstrapped with no regressions on x86_64-unknown-linux-gnu.  OK to
> commit?

OK.

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Mark Mitchell
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