libclc: Implement clz() builtin

Squashed commit of the following:

commit a0df0a0e86c55c1bdc0b9c0f5a739e5adef4b056
Author: Aaron Watry <awatry@gmail.com>
Date:   Mon Apr 15 18:42:04 2013 -0500

    libclc: Rename clz.ll to clz_if.ll to ensure it gets built.

    configure.py treats files that have the same name with the .cl and .ll
    extensions as overriding eachother.

    E.g. If you have clz.cl and clz.ll both specified to be built in the same
    SOURCES file, only the first file listed will actually be built.

    Since the contents of clz.ll were an interface that is implemented in
    clz_impl.ll, rename clz.ll to clz_if.ll to make sure that the interface is
    built.

commit 931b62bed05c58f737de625bd415af09571a6a5a
Author: Aaron Watry <awatry@gmail.com>
Date:   Sat Apr 13 12:32:54 2013 -0500

    libclc: llvm assembly implementation of clz

    Untested... currently crashes in the same manner as add_sat.

commit 6ef0b7b0b6d2e5584086b4b9a9243743b2e0538f
Author: Aaron Watry <awatry@gmail.com>
Date:   Sat Mar 23 12:35:27 2013 -0500

    libclc: Add stub clz builtin

    For scalar int/uint, attempt to use the clz llvm builtin.. for all others
    return 0 until an actual implementation is finished.

Patch by: Aaron Watry

llvm-svn: 185004