Port the whole userland to Clang.

This includes libraries, apps, kits, servers and add-ons. The only thing
missing is the kernel and kernel add-ons.

Instructions to build the Haiku userland with Clang:

 * Configure Haiku to build with gcc4.
 * Build the kernel, kernel add-ons and loader.
   (It might be easier to just build everything with gcc4 once and then
    delete the userland from generated/objects)
 * Locate i586-haiku-gcc in your toolchain. Put a symlink in the same
   directory called i586-haiku-clang, pointing to your host's Clang. You
   do not need to build Clang as a cross-compiler, as it is always a
   cross-compiler! If you get errors later on, try compiling Clang from
   trunk.
 * Edit generated/build/BuildConfig, replace i586-haiku-gcc with
   i586-haiku-clang.
 * Edit build/jam/BuildSetup. Look for HAIKU_WERRORFLAGS, there's a
   comment which flags to use for Clang. Uncomment it and comment out
   the flags in the line above.
 * Build Haiku the same way you would do with GCC4.

Known issues:

 * A few servers tend to crash on shutdown, as it seems gcc did not
   generate global dtors, which Clang does.