Fix: headers: make static inline upcasts compatible with C++

Fix: headers: make static inline upcasts compatible with C++

Issue
=====
There are many static inline functions which upcast a pointer from its
original type to another one known to be a parent. The functions upcast
with `(void *)` or `(const void *)`. While this works in C, it does not
in C++, which is more strict:

    In file included from install/include/babeltrace/babeltrace.h:75,
                     from test.cpp:1:

    install/include/babeltrace/trace-ir/field-class-const.h: In function
    ‘const bt_field_class_enumeration_mapping*
    bt_field_class_unsigned_enumeration_mapping_as_mapping_const(const
    bt_field_class_unsigned_enumeration_mapping*)’:

    install/include/babeltrace/trace-ir/field-class-const.h:100:9:
    error: invalid conversion from ‘const void*’ to ‘const
    bt_field_class_enumeration_mapping*’ [-fpermissive]

      return (const void *) mapping;

Solution
========
Use the new __BT_UPCAST() and __BT_UPCAST_CONST() macros to upcast
within headers. Those macros are defined in `babeltrace/types.h`. We use
static_cast<>() in C++ with the specific destination type, going through
`void *` or `const void *` to keep the same address.

Known drawbacks
===============
None.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Iaac8e55635eb2f358a546c32bd886a484f5ebfeb
Reviewed-on: https://review.gerrithub.io/c/eepp/babeltrace/+/452302
Reviewed-by: Simon Marchi <simon.marchi@polymtl.ca>