Add ErrorOr<>::getOrDie() which uses report_fatal_error instead of assert.

[alexfh]

Summary

This method allows writing code that consistently fails in case of
dereferencing an ErrorOr<> containing no value rather than crashing in debug and
using uninitialized data in release build.

[rnk]

I don't think we should be burying invisible calls to report_fatal_error in
libraries. Can you point at the code which wants to do this?

include/llvm/Support/ErrorOr.h

 ///
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_SUPPORT_ERROROR_H
 #define LLVM_SUPPORT_ERROROR_H
 
 #include "llvm/ADT/PointerIntPair.h"
 #include "llvm/Support/AlignOf.h"
+#include "llvm/Support/ErrorHandling.h"
 #include <cassert>
 #include <system_error>
 #include <type_traits>
 
 namespace llvm {
 template<class T, class V>
 typename std::enable_if< std::is_constructible<T, V>::value
                        , typename std::remove_reference<V>::type>::type &&
public:
   /// \brief Return false if there is an error.
   LLVM_EXPLICIT operator bool() const {
     return !HasError;
   }
 
   reference get() { return *getStorage(); }
   const_reference get() const { return const_cast<ErrorOr<T> >(this)->get(); }
 
+  reference getOrDie() {
+    if (HasError)
+      report_fatal_error("Cannot get value when an error exists!", false);
+    return *getStorage();
+  }
+  const_reference getOrDie() const {
+    return const_cast<ErrorOr<T>>(this)->getOrDie();
+  }
+
   std::error_code getError() const {
     return HasError ? *getErrorStorage() : std::error_code();
   }
 
   pointer operator ->() {
     return toPointer(getStorage());
   }