Add an LLVM_BUILTIN_DEBUGTRAP macro.

Summary:
This provides a macro that expands to __builtin_debugtrap() for clang,
and __debugbreak() for MSVC.

It intentionally expands to nothing for compilers that do not support a
similar mechanism that halts the debugger without otherwise crashing the
process.

Differential Revision: http://reviews.llvm.org/D18002

llvm-svn: 263095