Fixes the failure at https://lab.llvm.org/buildbot/#/builders/131/builds/62928,
and add comments about unused variable and update debugging output.

Coding my way home: 6.44615S, 128.16704W

Some maintenance on implementation and tests:
* Drop manual TargetMachineBuilder setup
* Drop addDebugSupport() in favor of Orc's enableDebuggerSupport()
* Check that debug support plugins append jit_code_entry
* Update and reduce sample input 

In preparation for removing the `#include "llvm/ADT/StringExtras.h"`
from the header to source file of `llvm/Support/Error.h`, first add in
all the missing includes that were previously included transitively
through this header.

Configure the plugin to automatically call the debugger rendezvous breakpoint `__jit_debug_register_code()` for every translation unit (enabled) or never at all (disabled). Default API and behavior remain unchanged.

If AutoRegisterCode is turned off, it's the client's own responsibility to call the rendezvous breakpoint function at an appropriate time.
Depending on the complexity of the debugger's rendezvous breakpoint implementation, this can provide significant performance improvements in cases where many debug objects are added in sequence.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D147310

Though this is a full port of the example, it is not yet fully functional due to a threading issue in the SimpleRemoteEPC implementation. The issue was discussed in D110530, but it needs a more thorough solution. For now we are dropping the dependency to the old `OrcRPC` here (it's been the last use-case in-tree). The test for the example is under review in ... and will be re-enabled once the threading issue is solved.

This was broken by the switch from JITTargetAddress to ExecutorAddr in
21a06254a3a.

The API change originated from D104694. The LLJITWithRemoteDebugging example and the test for it were disabled while it was in the works.