Give internal classes hidden visibility.

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

llvm-svn: 190534
diff --git a/llvm/lib/CodeGen/BranchFolding.h b/llvm/lib/CodeGen/BranchFolding.h
index 0d15ed7..9ca0f29 100644
--- a/llvm/lib/CodeGen/BranchFolding.h
+++ b/llvm/lib/CodeGen/BranchFolding.h
@@ -21,7 +21,7 @@
   class TargetInstrInfo;
   class TargetRegisterInfo;
 
-  class BranchFolder {
+  class LLVM_LIBRARY_VISIBILITY BranchFolder {
   public:
     explicit BranchFolder(bool defaultEnableTailMerge, bool CommonHoist);