bug#63251: 28.2; vhdl-mode contribution

[Mattias Engdegård]

> Following bug#62508, I've taken a bit of a closer look at the compiler
> regexp for ModelSim. I've modified it so that it can differentiate
> between Errors, Warnings and Notes when highlighting. I also make use of
> Xilinx Vivado at work, for which I've added a new compiler entry:

Sorry, but computer says no. The modified vhdl-compiler-alist value does not 
match its declared type.

You can easily check this by running `M-x customize-variable 
vhdl-compiler-alist`; it will show the contents as a single big text field and 
say (mismatch).
If the value matches the type, it would have shown the value structurally.

Another check is to run `make -C test test-custom-opts`, which checks the 
values of all defcustom variables in Emacs.

In this case it seems to be the `(2 . 3)` values at the end of the error 
message sublists. Remove them and the value matches the type.
If those (2 . 3) are important, perhaps you want to change the type instead but 
you would have to maintain compatibility somehow. I see there is a lot of code 
dealing with changes to the type of that variable in vhdl-mode.el.