[gdb/build, c++20] Fix Wdeprecated-enum-enum-conversion

When building gdb with clang 15 and -std=c++20, I run into:
...
gdbsupport/common-exceptions.h:203:32: error: arithmetic between different \
  enumeration types ('const enum return_reason' and 'const enum errors') is \
  deprecated [-Werror,-Wdeprecated-enum-enum-conversion]
    size_t result = exc.reason + exc.error;
                    ~~~~~~~~~~ ^ ~~~~~~~~~
...

Fix this by using to_underlying.

Likewise in a few other places.

Tested on x86_64-linux.