Currently, the availability of DSP instructions (ACLE 6.4.7) is handled in a hand-rolled tricky condition block in lib/Basic/Targets.cpp, with a FIXME: attached.
http://reviews.llvm.org/D12937 moves the handling of +t2dsp over to ARMTargetParser.def in LLVM, to be in line with other architecture extensions.
This is the corresponding patch to clang, to clear the FIXME: and update the tests.