[Driver/Frontend] Add -print-target-triple

Add a -print-target-triple command line option to the Swift frontend
and driver to allow other tools (e.g., SwiftPM) to query the host
triple as it is understood by the Swift compiler. This follows the
precedent set by Clang. Implements rdar://problem/57434967.

include/swift/Frontend/FrontendOptions.h

@@ -247,6 +247,10 @@ public:
   /// Indicates whether full help (including "hidden" options) should be shown.
   bool PrintHelpHidden = false;
 
+  /// Indicates that the frontend should print the target triple and then
+  /// exit.
+  bool PrintTargetTriple = false;
+
   /// Should we sort SIL functions, vtables, witness tables, and global
   /// variables by name when we print it out. This eases diffing of SIL files.
   bool EmitSortedSIL = false;