Add SystemRuntimeMetaCoroutine plugin to enable coroutine stack traces.

Summary:
When 'displayExtendedBacktrace' flag is set to true in the launch.json,
the coroutine stack trace will be added at the end of the normal stack
trace.

Can enable logs with:
```
(lldb) log enable lldb system-runtime
```

Test Plan:
Manually tested with admarket/adfinder:adfinder target. Retrieving an async stack trace of size 5-10 takes around 5-7ms

 {F1967331044}

Reviewers: jeffreytan, gclayton

Reviewed By: jeffreytan

Subscribers: #lldb_team

Differential Revision: https://phabricator.intern.facebook.com/D66459927

Author: Miro Bucko

lldb/source/Plugins/SystemRuntime/CMakeLists.txt

@@ -2,3 +2,4 @@ set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND SystemRuntime)
 set_property(DIRECTORY PROPERTY LLDB_TOLERATED_PLUGIN_DEPENDENCIES TypeSystem)
 
 add_subdirectory(MacOSX)
+add_subdirectory(MetaCoroutine)

lldb/source/Plugins/SystemRuntime/MetaCoroutine/CMakeLists.txt

@@ -0,0 +1,3 @@
+add_lldb_library(lldbPluginSystemRuntimeMetaCoroutine PLUGIN
+  SystemRuntimeMetaCoroutine.cpp
+  )