Always pass a diagnostic handler to the linker.

Before this patch the diagnostic handler was optional. If it was not
passed, the one in the LLVMContext was used.

That is probably not a pattern we want to follow. If each area has an
optional callback, there is a sea of callbacks and it is hard to follow
which one is called.

Doing this also found cases where the callback is a nice addition, like
testing that no errors or warnings are reported.

The other option is to always use the diagnostic handler in the
LLVMContext. That has a few problems

* To implement the C API we would have to set the diag handler and then
  set it back to the original value.
* Code that creates the context might be far away from code that wants
  the diagnostics.

I do have a patch that implements the second option and will send that as
an RFC.

llvm-svn: 254777
diff --git a/llvm/tools/bugpoint/BugDriver.cpp b/llvm/tools/bugpoint/BugDriver.cpp
index 39887d5..9edc242 100644
--- a/llvm/tools/bugpoint/BugDriver.cpp
+++ b/llvm/tools/bugpoint/BugDriver.cpp
@@ -15,6 +15,7 @@
 
 #include "BugDriver.h"
 #include "ToolRunner.h"
+#include "llvm/IR/DiagnosticPrinter.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/Verifier.h"
 #include "llvm/IRReader/IRReader.h"
@@ -112,6 +113,12 @@
   return Result;
 }
 
+static void diagnosticHandler(const DiagnosticInfo &DI) {
+  DiagnosticPrinterRawOStream DP(errs());
+  DI.print(DP);
+  errs() << '\n';
+}
+
 // This method takes the specified list of LLVM input files, attempts to load
 // them, either as assembly or bitcode, then link them together. It returns
 // true on failure (if, for example, an input bitcode file could not be
@@ -132,7 +139,7 @@
     if (!M.get()) return true;
 
     outs() << "Linking in input file: '" << Filenames[i] << "'\n";
-    if (Linker::linkModules(*Program, *M))
+    if (Linker::linkModules(*Program, *M, diagnosticHandler))
       return true;
   }