Patches from vtk (#358)

* Drop the export attribute on forward declaration

They don't mean anything on forward declarations anyways.

* Fixed -Wunused-parameter warnings in some C++ files

Allowed all the attribute #defines to work in C++ also.

* Fixed -Wunused-variable warnings by removing dead code

* Fixed all -Wshorten-64-to-32 warnings by adjusting casts

There was no truncation in fact, since the unsigned result was upcast to size_t then implicitly downcast to uint32_t.

* Fixed all -Wcomma warnings by spliting statements onto own lines

* Fixed all -Wself-assign warnings with different unused param suppression

* Fixed all -Wextra-semi warnings

* Fixed all -Wswitch-enum warnings

Just added new explicit cases with the existing default case.

* Fixed all -Wmissing-prototypes warnings

Just made functions static. Moved them into an extern "C" block.

* Reformatted source with bin/format_source using clang version 10.0.1

Co-authored-by: Larry Knox <lrknox@hdfgroup.org>

c++/src/H5Attribute.cpp

@@ -41,7 +41,7 @@ namespace H5 {
 using std::cerr;
 using std::endl;
 
-class H5_DLLCPP H5Object; // forward declaration for UserData4Aiterate
+class H5Object; // forward declaration for UserData4Aiterate
 
 //--------------------------------------------------------------------------
 // Function:    Attribute default constructor