ELF: Make link() to take an output stream to which error messages are written.

http://reviews.llvm.org/D16668

llvm-svn: 259597
diff --git a/lld/ELF/Error.cpp b/lld/ELF/Error.cpp
index e3add1b..4a5b8af 100644
--- a/lld/ELF/Error.cpp
+++ b/lld/ELF/Error.cpp
@@ -16,11 +16,12 @@
 namespace elf2 {
 
 bool HasError;
+llvm::raw_ostream *ErrorOS;
 
 void warning(const Twine &Msg) { llvm::errs() << Msg << "\n"; }
 
 void error(const Twine &Msg) {
-  llvm::errs() << Msg << "\n";
+  *ErrorOS << Msg << "\n";
   HasError = true;
 }