[Support] Explicitly instantiate BumpPtrAllocatorImpl

Most clients only ever use the default BumpPtrAllocator.

llvm/include/llvm/Support/Allocator.h

@@ -434,6 +434,8 @@ private:
   template <typename T> friend class SpecificBumpPtrAllocator;
 };
 
+extern template class BumpPtrAllocatorImpl<>;
+
 /// The standard BumpPtrAllocator which just uses the default template
 /// parameters.
 typedef BumpPtrAllocatorImpl<> BumpPtrAllocator;

llvm/lib/Support/Allocator.cpp

@@ -15,6 +15,8 @@
 
 namespace llvm {
 
+template class BumpPtrAllocatorImpl<>;
+
 namespace detail {
 
 void printBumpPtrAllocatorStats(unsigned NumSlabs, size_t BytesAllocated,