[Driver] Add method to redirect output of Compilation.

[nhaustov]

Summary

Currently output of child process always goes to same stdout/stderr,
however in my use case, it needs to be captured and presented to the user.

Add Redirect method to Compilation and use existing infrastructure
for redirecting output of commands.

[nhaustov]

Fix summary.

[nhaustov]

Ping.

include/clang/Driver/Compilation.h

public:
 
   /// initCompilationForDiagnostics - Remove stale state and suppress output
   /// so compilation can be reexecuted to generate additional diagnostic
   /// information (e.g., preprocessed source(s)).
   void initCompilationForDiagnostics();
 
   /// Return true if we're compiling for diagnostics.
   bool isForDiagnostics() const { return ForDiagnostics; }
+
+  /// RedirectOutput - Redirect output of this compilation. Can only be done once.
+  ///
+  /// \param Redirects - array of pointers to paths. The array
+  /// should have a size of three. The inferior process's
+  /// stdin(0), stdout(1), and stderr(2) will be redirected to the
+  /// corresponding paths. This compilation instance becomes
+  /// the owner of Redirects and will delete the array and StringRef's.
+  void Redirect(const StringRef** Redirects);
 };
 
 } // end namespace driver
 } // end namespace clang
 
 #endif

lib/Driver/Compilation.cpp

Compilation::~Compilation() {
   for (llvm::DenseMap<std::pair<const ToolChain*, const char*>,
                       DerivedArgList*>::iterator it = TCArgs.begin(),
          ie = TCArgs.end(); it != ie; ++it)
     if (it->second != TranslatedArgs)
       delete it->second;
 
   // Free redirections of stdout/stderr.
   if (Redirects) {
+    delete Redirects[0];
     delete Redirects[1];
     delete Redirects[2];
     delete [] Redirects;
   }
 }
 
 const DerivedArgList &Compilation::getArgsForToolChain(const ToolChain *TC,
                                                        const char *BoundArch) {
void Compilation::initCompilationForDiagnostics() {
   Redirects[0] = nullptr;
   Redirects[1] = new StringRef();
   Redirects[2] = new StringRef();
 }
 
 StringRef Compilation::getSysRoot() const {
   return getDriver().SysRoot;
 }
+
+void Compilation::Redirect(const StringRef** Redirects) {
+  this->Redirects = Redirects;
+}