Give internal classes hidden visibility.

Worth 100k on a linux/x86_64 Release+Asserts clang.

llvm-svn: 190534
diff --git a/llvm/lib/Transforms/Instrumentation/DebugIR.h b/llvm/lib/Transforms/Instrumentation/DebugIR.h
index 13774cf..06fea48 100644
--- a/llvm/lib/Transforms/Instrumentation/DebugIR.h
+++ b/llvm/lib/Transforms/Instrumentation/DebugIR.h
@@ -21,7 +21,7 @@
 
 namespace llvm {
 
-class DebugIR : public llvm::ModulePass {
+class LLVM_LIBRARY_VISIBILITY DebugIR : public llvm::ModulePass {
   /// If true, write a source file to disk.
   bool WriteSourceToDisk;