[LTO] Add error message on IO error in compileOptimizedToFile.

(No testcase because it's difficult to force an error here.)

Differential Revision: https://reviews.llvm.org/D26371



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@286177 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed