ENH: Update LLVM/Clang ExternalProject to 3.6.0.
ENH: Update LLVM/Clang ExternalProject to 3.6.0.

Also use the official upstream tarballs. This removes the subversion dependency.

We used tarballs recompressed from upstream in a gzip format to improve
compatibility with CMake < 3.1 since upstream uses xzip.

Change-Id: Iaf4e12892ebc91d0ab319b49056fcfda436bde3d

stnava/ITK,LucasGandel/ITK,fedral/ITK,stnava/ITK,richardbeare/ITK,InsightSoftwareConsortium/ITK,zachary-williamson/ITK,atsnyder/ITK,zachary-williamson/ITK,jcfr/ITK,PlutoniumHeart/ITK,LucasGandel/ITK,vfonov/ITK,stnava/ITK,LucHermitte/ITK,zachary-williamson/ITK,msmolens/ITK,atsnyder/ITK,zachary-williamson/ITK,thewtex/ITK,hjmjohnson/ITK,msmolens/ITK,blowekamp/ITK,richardbeare/ITK,atsnyder/ITK,LucHermitte/ITK,BRAINSia/ITK,jmerkow/ITK,spinicist/ITK,zachary-williamson/ITK,stnava/ITK,richardbeare/ITK,hjmjohnson/ITK,vfonov/ITK,jcfr/ITK,BlueBrain/ITK,PlutoniumHeart/ITK,Kitware/ITK,jcfr/ITK,hjmjohnson/ITK,LucasGandel/ITK,LucasGandel/ITK,spinicist/ITK,BRAINSia/ITK,fedral/ITK,fbudin69500/ITK,BlueBrain/ITK,spinicist/ITK,jcfr/ITK,atsnyder/ITK,blowekamp/ITK,fedral/ITK,PlutoniumHeart/ITK,LucHermitte/ITK,ajjl/ITK,InsightSoftwareConsortium/ITK,InsightSoftwareConsortium/ITK,InsightSoftwareConsortium/ITK,atsnyder/ITK,BRAINSia/ITK,blowekamp/ITK,BRAINSia/ITK,fbudin69500/ITK,Kitware/ITK,hjmjohnson/ITK,hjmjohnson/ITK,hjmjohnson/ITK,vfonov/ITK,BlueBrain/ITK,richardbeare/ITK,fbudin69500/ITK,thewtex/ITK,PlutoniumHeart/ITK,fedral/ITK,fbudin69500/ITK,spinicist/ITK,fedral/ITK,ajjl/ITK,LucHermitte/ITK,ajjl/ITK,LucasGandel/ITK,spinicist/ITK,biotrump/ITK,BlueBrain/ITK,malaterre/ITK,thewtex/ITK,atsnyder/ITK,blowekamp/ITK,jmerkow/ITK,blowekamp/ITK,BlueBrain/ITK,vfonov/ITK,vfonov/ITK,spinicist/ITK,spinicist/ITK,LucasGandel/ITK,PlutoniumHeart/ITK,stnava/ITK,msmolens/ITK,thewtex/ITK,ajjl/ITK,ajjl/ITK,thewtex/ITK,biotrump/ITK,PlutoniumHeart/ITK,msmolens/ITK,zachary-williamson/ITK,msmolens/ITK,vfonov/ITK,biotrump/ITK,jmerkow/ITK,BlueBrain/ITK,LucHermitte/ITK,LucHermitte/ITK,fedral/ITK,jmerkow/ITK,PlutoniumHeart/ITK,InsightSoftwareConsortium/ITK,malaterre/ITK,jcfr/ITK,stnava/ITK,jcfr/ITK,ajjl/ITK,biotrump/ITK,fbudin69500/ITK,LucasGandel/ITK,BlueBrain/ITK,BRAINSia/ITK,msmolens/ITK,Kitware/ITK,ajjl/ITK,atsnyder/ITK,hjmjohnson/ITK,atsnyder/ITK,vfonov/ITK,zachary-williamson/ITK,vfonov/ITK,malaterre/ITK,malaterre/ITK,zachary-williamson/ITK,BRAINSia/ITK,richardbeare/ITK,Kitware/ITK,LucHermitte/ITK,thewtex/ITK,PlutoniumHeart/ITK,stnava/ITK,spinicist/ITK,stnava/ITK,fbudin69500/ITK,Kitware/ITK,thewtex/ITK,spinicist/ITK,msmolens/ITK,zachary-williamson/ITK,malaterre/ITK,ajjl/ITK,malaterre/ITK,Kitware/ITK,blowekamp/ITK,richardbeare/ITK,jcfr/ITK,vfonov/ITK,jmerkow/ITK,LucHermitte/ITK,fedral/ITK,biotrump/ITK,atsnyder/ITK,blowekamp/ITK,Kitware/ITK,BRAINSia/ITK,blowekamp/ITK,LucasGandel/ITK,fbudin69500/ITK,richardbeare/ITK,InsightSoftwareConsortium/ITK,BlueBrain/ITK,malaterre/ITK,malaterre/ITK,stnava/ITK,malaterre/ITK,msmolens/ITK,jmerkow/ITK,InsightSoftwareConsortium/ITK,fedral/ITK,biotrump/ITK,jcfr/ITK,biotrump/ITK,biotrump/ITK,jmerkow/ITK,jmerkow/ITK,fbudin69500/ITK