This patch is to fix radar://8426430. It is about llvm support of builtin_debugtrap()
which is supposed to consistently raise SIGTRAP across all systems. In contrast,
builtin_trap() behave differently on different systems. e.g. it raises SIGTRAP on ARM, and
SIGILL on X86. The purpose of builtin_debugtrap() is to consistently provide "trap"
functionality, in the mean time preserve the compatibility with on gcc on builtin_trap().
The X86 backend is already able to handle debugtrap(). This patch is to: 1) make front-end recognize "__builtin_debugtrap()" (emboddied in the one-line change to Clang). 2) In DAG legalization phase, by default, "debugtrap" will be replaced with "trap", which make the __builtin_debugtrap() "available" to all existing ports without the hassle of changing their code. 3) If trap-function is specified (via -trap-func=xyz to llc), both __builtin_debugtrap() and __builtin_trap() will be expanded into the function call of the specified trap function. This behavior may need change in the future. The provided testing-case is to make sure 2) and 3) are working for ARM port, and we
already have a testing case for x86.