[dump] Remove NDEBUG from test to enable dump methods [NFC]

Summary:
Add LLVM_FORCE_ENABLE_DUMP cmake option, and use it along with
LLVM_ENABLE_ASSERTIONS to set LLVM_ENABLE_DUMP.

Remove NDEBUG and only use LLVM_ENABLE_DUMP to enable dump methods.

Move definition of LLVM_ENABLE_DUMP from config.h to llvm-config.h so
it'll be picked up by public headers.

Differential Revision: https://reviews.llvm.org/D38406

llvm-svn: 315590
diff --git a/llvm/lib/Transforms/Scalar/GVN.cpp b/llvm/lib/Transforms/Scalar/GVN.cpp
index a642eec..1ac2bea 100644
--- a/llvm/lib/Transforms/Scalar/GVN.cpp
+++ b/llvm/lib/Transforms/Scalar/GVN.cpp
@@ -646,7 +646,7 @@
   return PA;
 }
 
-#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
+#ifdef LLVM_ENABLE_DUMP
 LLVM_DUMP_METHOD void GVN::dump(DenseMap<uint32_t, Value*>& d) const {
   errs() << "{\n";
   for (DenseMap<uint32_t, Value*>::iterator I = d.begin(),