Automated porting of druntime to new platforms using libclang

[Joakim]

I've been porting dmd to Android/x86 recently and I've been 
thinking about whether there's an easier way to handle such 
porting in the future.  The vast majority of my druntime patches 
simply find the equivalent function or macro in the Android C 
library, bionic.  This is just a lot of name and parameter type 
checking most of the time: couldn't this be automated for new 
platforms with libclang?

Write a D utility that finds all the platform blocks in druntime 
and calls libclang to parse the headers and translate the results 
into D, like htod does, then inserts the new version blocks into 
druntime, which you can then quickly manually check.  Perhaps 
this can be built on top of dstep, which I just found while 
googling about htod for this post: :)

https://github.com/jacob-carlborg/dstep

Obviously no utility will catch everything, for example, it's not 
going to know how to replace missing functions.  I'm not sure if 
libclang provides for a way to handle architecture-specific 
declarations either.  But it will save a bunch of time for 
druntime porters, and even catch bugs in the existing manually 
ported platforms for druntime.  Of course, it will only work on 
existing clang platforms, but I doubt we're going to beat clang 
to a platform. ;)

It could also provide a way to separate out glibc from linux, as 
we've been talking about on my Android stdc pull request.  After 
checking the relevant C header against druntime, this utility 
could query the local package manager, pacman in the case of Arch 
linux, to see what package owned that C header and then separate 
glibc from linux version blocks by using that info.

I don't have any experience with libclang, so I thought I'd run 
the idea by you all first.  Let me know if you see any problems 
with this idea.