Fix documentation for TargetOptions.

[johannes]

Summary

See https://llvm.org/PR5935
It has been years, apparently it is not of popular demand to set a default
triple, so I suggest to change the documentation accordingly.

[v.g.vassilev]

I think the second sentence is important.

[v.g.vassilev]

Sorry, ignore my previous comment. LGTM!

[v.g.vassilev]

Landed in r302521.

include/clang/Basic/TargetOptions.h

 #include <vector>
 #include "clang/Basic/OpenCLOptions.h"
 
 namespace clang {
 
 /// \brief Options for controlling the target.
 class TargetOptions {
 public:
-  /// If given, the name of the target triple to compile for. If not given the
-  /// target will be selected to match the host.
+  /// The name of the target triple to compile for.
   std::string Triple;
 
   /// When compiling for the device side, contains the triple used to compile
   /// for the host.
   std::string HostTriple;
 
   /// If given, the name of the target CPU to generate code for.
   std::string CPU;