Finally bite the bullet and make the major change: split the clang namespace

out of the llvm namespace.  This makes the clang namespace be a sibling of
llvm instead of being a child.

The good thing about this is that it makes many things unambiguous.  The
bad things is that many things in the llvm namespace (notably data structures
like smallvector) now require an llvm:: qualifier.  IMO, libsystem and libsupport
should be split out of llvm into their own namespace in the future, which will fix
this issue.

llvm-svn: 39659