[PATCH] Extend diagnostics in increment/decrement check

Hello,

This patch extends the increment/decrement check diagnostics in Sema to give a more suitable error when incrementing/decrementing a non MLV.

sema_const_mlv_error.patch (4.6 KB)

This patch seems fairly specific to only one particular case. As an
alternative could we factor out the code in CheckAssignmentOperands so
we would handle all cases in one place?

- Daniel