[PATCH] Don't emit prologues/epilogues for naked functions (PR18791, PR20028)

[Saleem Abdulrasool]

Is supporting the __attribute__ (( naked )) spelling on x86 intentional?  It can certainly be a nice extension, but keep in mind that GCC only permits this on ARM, AVR, MCORE, MSP430, NDS32, RL78, RX and SPU ports.  However, we need the __declspec(naked) spelling for MSVC compatibility.  It might be nice to document that we support the attribute across the board rather than keep GCC compatibility.

http://reviews.llvm.org/D5183