Fix build failure on Win32

Commit ed51438 breaks building on Win32, since Standard_Size
and Standard_Utf32Char define the same types.
Modify Standard_Integer.hxx to add overloaded functions only
when these types are different.
Drop ifdef guard to fix a build failure with MSVC

We added the ifdef guard in d5b9cbb because MSVC was
smart enough to find that the return statement could
not be reached; this method has been modified in OCCT
6.6.0 and now the return statement is needed.

This commit fix issue #389.
Merge pull request #388 from tpaviot/review/fix-build-win32

Fix build failure on Win32