[lldb] Move and clean-up the Declaration class (NFC)

This patch moves the Declaration class from the Symbol library to the
Core library. This will allow to use it in a more generic fashion and
aims to lower the dependency cycles when it comes to the linking.

The patch also does some cleaning up by making column information
permanent and removing the LLDB_ENABLE_DECLARATION_COLUMNS directives.

Differential revision: https://reviews.llvm.org/D101556



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>