Compiler: Remove empty fallback definition for LLVM_BUILTIN_UNREACHABLE

[dexonsmith]

Summary

llvm_unreachable() and LLVM_ASSUME_ALIGNED use
defined(LLVM_BUILTIN_UNREACHABLE) to check whether it has a
definition. Remove the fallback added in 26827337dff26ba3 (as a drive-by
when updating the GCC logic) and add a comment to prevent future
mistakes.

[dexonsmith]

Alternatively, maybe we don't support building with compilers that don't have a builtin unreachable anymore, and can have:

#else
#error

and we can simplify logic that depends on this macro... but it seems bit friendlier to keep this alive since it doesn't seem hard.

[dexonsmith]

Thanks for the review! Landed in 892c104fb71b86dc6399e36a82ae920e00ccae17.

In D122167#3397263, @dexonsmith wrote:

Alternatively, maybe we don't support building with compilers that don't have a builtin unreachable anymore, and can have:

#else
#error

and we can simplify logic that depends on this macro... but it seems bit friendlier to keep this alive since it doesn't seem hard.

(Can always do that as a follow-up if it makes sense.)

llvm/include/llvm/Support/Compiler.h

 // LLVM_ATTRIBUTE_DEPRECATED(decl, "message")
 // This macro will be removed.
 // Use C++14's attribute instead: [[deprecated("message")]]
 #define LLVM_ATTRIBUTE_DEPRECATED(decl, message) [[deprecated(message)]] decl
 
 /// LLVM_BUILTIN_UNREACHABLE - On compilers which support it, expands
 /// to an expression which states that it is undefined behavior for the
 /// compiler to reach this point.  Otherwise is not defined.
+///
+/// '#else' is intentionally left out so that other macro logic (e.g.,
+/// LLVM_ASSUME_ALIGNED and llvm_unreachable()) can detect whether
+/// LLVM_BUILTIN_UNREACHABLE has a definition.
 #if __has_builtin(__builtin_unreachable) || defined(__GNUC__)
 # define LLVM_BUILTIN_UNREACHABLE __builtin_unreachable()
 #elif defined(_MSC_VER)
 # define LLVM_BUILTIN_UNREACHABLE __assume(false)
-#else
-# define LLVM_BUILTIN_UNREACHABLE
 #endif
 
 /// LLVM_BUILTIN_TRAP - On compilers which support it, expands to an expression
 /// which causes the program to exit abnormally.
 #if __has_builtin(__builtin_trap) || defined(__GNUC__)
 # define LLVM_BUILTIN_TRAP __builtin_trap()
 #elif defined(_MSC_VER)
 // The __debugbreak intrinsic is supported by MSVC, does not require forward