r250577 - [modules] Allow the error when explicitly loading an incompatible module file

[Brad King via cfe-commits]

On 10/20/2015 04:38 AM, Manuel Klimek wrote:
> On Tue, Oct 20, 2015 at 5:52 AM Sean Silva wrote:
>> get cmake to generate clang module map files and add explicit module build steps?
> 
> I have some experience hacking on cmake, and from my experience I think
> this shouldn't be too hard to get working (mainly work ;)

I agree this shouldn't be too hard on the CMake side.  Manuel, please
come over to the cmake dev list to raise the design discussion.  Then
we can guide your implementation work.  The main design challenges
I foresee are:

1. Deciding how this behavior should be activated for a project by
   its code and/or by the user.

2. Selection of the proper set of headers if it is not exactly the set
   listed in the target for some reason.  Might this ever by more
   granular than a whole library target?

3. Finding the right place during the CMake generation process to add
   the rules for this.

We already detect the Clang compiler version so deciding if it is
new enough to support the feature should not be hard.

Thanks,
-Brad