Add "process metadata" Mach-O LC_NOTE for corefiles

Add a new LC_NOTE for Mach-O corefiles, "proces metadata", which is a
JSON string.  Currently there may be a `threads` key in the JSON,
and if `threads` is present, it is an array with the same number of
elements as there are LC_THREADs in the corefile.  This patch adds
support for a `thread_id` key-value for each `thread` entry, to
supply a thread ID for that LC_THREAD.

Differential Revision: https://reviews.llvm.org/D158785
rdar://113037252

(cherry picked from commit 2cab996)

Author: jasonmolenda

lldb/include/lldb/Symbol/ObjectFile.h

@@ -561,6 +561,30 @@ class ObjectFile : public std::enable_shared_from_this<ObjectFile>,
     return false;
   }
 
+  /// Get metadata about threads from the corefile.
+  ///
+  /// The corefile may have metadata (e.g. a Mach-O "thread extrainfo"
+  /// LC_NOTE) which for the threads in the process; this method tries
+  /// to retrieve them.
+  ///
+  /// \param[out] tids
+  ///     Filled in with a vector of tid_t's that matches the number
+  ///     of threads in the corefile (ObjectFile::GetNumThreadContexts).
+  ///     If a tid is not specified for one of the corefile threads,
+  ///     that entry in the vector will have LLDB_INVALID_THREAD_ID and
+  ///     the caller should assign a tid to the thread that does not
+  ///     conflict with the ones provided in this array.
+  ///     As additional metadata are added, this method may return a
+  ///     \a tids vector with no thread id's specified at all; the
+  ///     corefile may only specify one of the other metadata.
+  ///
+  /// \return
+  ///     Returns true if thread metadata was found in this corefile.
+  ///
+  virtual bool GetCorefileThreadExtraInfos(std::vector<lldb::tid_t> &tids) {
+    return false;
+  }
+
   virtual lldb::RegisterContextSP
   GetThreadContextAtIndex(uint32_t idx, lldb_private::Thread &thread) {
     return lldb::RegisterContextSP();