Add an LLVM_BUILTIN_DEBUGTRAP macro.
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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@263095 91177308-0d34-0410-b5e6-96231b3b80d8

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