[lldb/Utility] Use assert instead of llvm_unreachable for LLDBAssert

llvm_unreachable is marked noreturn so the compiler can assume the code
for printing the error message in release builds isn't hit which defeats
the purpose.

lldb/source/Utility/LLDBAssert.cpp

@@ -21,10 +21,10 @@ void lldb_private::lldb_assert(bool expression, const char *expr_text,
   if (LLVM_LIKELY(expression))
     return;
 
-  // In a Debug configuration lldb_assert() behaves like assert(0).
-  llvm_unreachable("lldb_assert failed");
+  // If asserts are enabled abort here.
+  assert(false && "lldb_assert failed");
 
-  // In a Release configuration it will print a warning and encourage the user
+  // In a release configuration it will print a warning and encourage the user
   // to file a bug report, similar to LLVM’s crash handler, and then return
   // execution.
   errs() << format("Assertion failed: (%s), function %s, file %s, line %u\n",