Drop non-cfi assembly support from clang.

After this patch clang will ignore -fdwarf2-cfi-asm and -ffno-dwarf2-cfi-asm and
always print assembly that uses cfi directives.

In llvm, MC itself supports cfi since the end of 2010 (support started
in r119972, is reported in the 2.9 release notes).

In binutils the support has been around for much longer. It looks like
support started to be added in May 2003. It is available in 2.15
(31-Aug-2011, 2.14 is from 12-Jun-2003).