[ORC-RT] Add a comment explaining the purpose of this testcase. NFC.

(cherry picked from commit aefa30e)

Author: lhames

compiler-rt/test/orc/TestCases/Darwin/Generic/exceptions.cpp

@@ -3,6 +3,7 @@
 //
 // REQUIRES: system-darwin && host-arch-compatible
 
+// Test that trivial throw / catch works.
 int main(int argc, char *argv[]) {
   try {
     throw 42;