PSA: GHC can now be built with Clang

[Austin Seipp]

Hi all,

As of commit 5dc74f it should now be possible to build a working
stage1 and stage2 compiler with (an extremely recent) Clang. With some
caveats.

You can just do:

$ CC=/path/to/clang ./configure --with-gcc=/path/to/clang
$ make

I have done this work on Linux. I don't expect much difficulty on Mac
OS X, but it needs testing. Ditto with Windows, although Clang/mingw
is considered experimental.

The current caveats are:

 * The testsuite will probably fail everywhere, because of some
warnings that happen during the linking phase when you invoke the
built compiler. So the testsuite runner will probably be unhappy.
Clang is very noisy about unused options, unlike GCC. That needs to be
fixed somewhere in DriverPipeline I'd guess, but with some
refactoring.
 * Some of the stage2 libraries don't build due to a Clang bug. These
are vector/primitive/dph so far.
 * There is no buildbot or anything to cover it.

You will need a very recent Clang. Due to this bug (preventing
primitive etc from building,) you'll preferably want to use an SVN
checkout from about 6 hours ago at latest:

http://llvm.org/bugs/show_bug.cgi?id=16363

Hilariously, this bug was tripped on primitive's Data.Primitive.Types
module due to some CPP weirdness. But even with a proper bugfix and no
segfault, it still fails to correctly parse this same module with the
same CPP declarations. I'm fairly certain this is another bug in
Clang, but I might be wrong. I'm trying to isolate it. Unfortunately
Clang/LLVM 3.3 was just released and it won't see bugfix releases. But
it will *probably* work if we just get rid of the CPP tomfoolery in
primitive. I'll be testing it in the next few days to see if we can
get 3.3 supported. (I'm sort of kicking myself in the head for not
doing this a week or two ago...)

Anyway, there are some rough edges but it should be in shape for 7.8 I
hope. It should be especially welcome for Mac users. (I'm also hoping
modern Macs could even go all-clang-all-the-time if my fix for #7602
can go in soon...)

PS. If you use ASSERT, I just fixed a lot of instances of using that
macro, involving whitespace between the name and arguments (commit
d8ee2b.) Clang does not forgive you for this. Should I note this
anywhere for the future in the wiki or something?

-- 
Regards,
Austin - PGP: 4096R/0x91384671