Fix "settings set -g" so it works again.

[clayborg]

Summary

When we switched options over to use the Options.td file, a bug was introduced that caused the "-g" option for "settings set" to require a filename arguemnt. This patch fixes this issue and adds a test so this doesn't regress.

[JDevlieghere]

LGTM

lldb/source/Commands/Options.td

def help_hide_aliases : Option<"hide-aliases", "a">,
     Desc<"Hide aliases in the command list.">;
   def help_hide_user : Option<"hide-user-commands", "u">,
     Desc<"Hide user-defined commands from the list.">;
   def help_show_hidden : Option<"show-hidden-commands", "h">,
     Desc<"Include commands prefixed with an underscore.">;
 }
 
 let Command = "settings set" in {
-  def setset_global : Option<"global", "g">, Arg<"Filename">,
-    Completion<"DiskFile">,
+  def setset_global : Option<"global", "g">,
     Desc<"Apply the new value to the global default value.">;
   def setset_force : Option<"force", "f">,
     Desc<"Force an empty value to be accepted as the default.">;
 }
 
 let Command = "settings write" in {
   def setwrite_file : Option<"file", "f">, Required, Arg<"Filename">,
     Completion<"DiskFile">,

lldb/test/Shell/Settings/TestSettingsSet.test

 # This tests setting setting values.
 
 # Check that setting an empty value with -f(orce) clears the value.
 # RUN: not %lldb -b -s %s 2>&1 | FileCheck %s
 
+# Make sure that "settings set -g" no longer requires a bogus filename.
+settings set -g target.skip-prologue false
+settings show target.skip-prologue
+# CHECK: target.skip-prologue (boolean) = false
+
 settings set tab-size 16
 settings show tab-size
 # CHECK: tab-size (unsigned) = 16
 
 settings set -f tab-size
 settings show tab-size
 # CHECK: tab-size (unsigned) = 4
 
 settings set tab-size
 # CHECK: error: 'settings set' takes more arguments