Add an LLVM_BUILTIN_DEBUGTRAP macro.

[rudkx]

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.

[rudkx]

I don't think this ever went out to llvm-commits, so I'm going to re-open with llvm-commits as a subscriber when opening.

include/llvm/Support/Compiler.h

 // declarations involving platform-specific typedefs (unlike RaiseException),
 // results in a call to vectored exception handlers, and encodes to a short
 // instruction that still causes the trapping behavior we want.
 # define LLVM_BUILTIN_TRAP __debugbreak()
 #else
 # define LLVM_BUILTIN_TRAP *(volatile int*)0x11 = 0
 #endif
 
+/// LLVM_BUILTIN_DEBUGTRAP - On compilers which support it, expands to
+/// an expression which causes the program to break while running
+/// under a debugger.
+#if __has_builtin(__builtin_debugtrap)
+# define LLVM_BUILTIN_DEBUGTRAP __builtin_debugtrap()
+#elif defined(_MSC_VER)
+// The __debugbreak intrinsic is supported by MSVC and breaks while
+// running under the debugger, and also supports invoking a debugger
+// when the OS is configured appropriately.
+# define LLVM_BUILTIN_DEBUGTRAP __debugbreak()
+#else
+// Just continue execution when built with compilers that have no
+// support. This is a debugging aid and not intended to force the
+// program to abort if encountered.
+# define LLVM_BUILTIN_DEBUGTRAP
+#endif
+
 /// \macro LLVM_ASSUME_ALIGNED
 /// \brief Returns a pointer with an assumed alignment.
 #if __has_builtin(__builtin_assume_aligned) || LLVM_GNUC_PREREQ(4, 7, 0)
 # define LLVM_ASSUME_ALIGNED(p, a) __builtin_assume_aligned(p, a)
 #elif defined(LLVM_BUILTIN_UNREACHABLE)
 // As of today, clang does not support __builtin_assume_aligned.
 # define LLVM_ASSUME_ALIGNED(p, a) \
            (((uintptr_t(p) % (a)) == 0) ? (p) : (LLVM_BUILTIN_UNREACHABLE, (p)))