Revert "Give internal classes hidden visibility."

It works with clang, but GCC has different rules so we can't make all of those
hidden. This reverts commit r190534.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190536 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Instrumentation/DebugIR.h

@@ -21,7 +21,7 @@
 
 namespace llvm {
 
-class LLVM_LIBRARY_VISIBILITY DebugIR : public llvm::ModulePass {
+class DebugIR : public llvm::ModulePass {
   /// If true, write a source file to disk.
   bool WriteSourceToDisk;