[lldb] Add debugger.external-editor setting

Add a new setting (debugger.external-editor) to specify an external
editor. The setting takes precedence over the existing
LLDB_EXTERNAL_EDITOR environment variable.

Differential revision: https://reviews.llvm.org/D149565
diff --git a/lldb/source/Core/CoreProperties.td b/lldb/source/Core/CoreProperties.td
index 6272992..ccee2a6 100644
--- a/lldb/source/Core/CoreProperties.td
+++ b/lldb/source/Core/CoreProperties.td
@@ -135,6 +135,10 @@
     Global,
     DefaultFalse,
     Desc<"Whether to use an external editor or not.">;
+  def ExternalEditor: Property<"external-editor", "String">,
+    Global,
+    DefaultStringValue<"">,
+    Desc<"External editor to use when use-external-editor is enabled.">;
   def UseColor: Property<"use-color", "Boolean">,
     Global,
     DefaultTrue,