Handle Ada Pragma Import and Pragma Export

Ada can import C APIs and also export Ada constructs to C via Pragma
Import and Pragma Export.  This patch adds support for these to gdb,
by arranging to either defer some aspects of a symbol to the
underlying C symbol (for Import) or by introducing a second symbol
(for Export).  A somewhat tricky approach is needed, both because gdb
doesn't generally handle symbol aliasing, and because Ada treats
symbol names in an unusual way (as compared to the rest of gdb).