Fix regression due to Pragma Import series

A co-worker here at AdaCore discovered that the Pragma Import series
caused a rgression.  When debugging gnat1, gdb started asking for
overload resolution like:

(gdb) call pp(n)
Multiple matches for pp
[0] cancel
[1] pp (types.union_id) at ../../gcc/gcc/ada/treepr.adb:511
[2] treepr.pp (types.union_id) at ../../gcc/gcc/ada/treepr.adb:511

This worked before the series, and is strange anyway, because the
matches refer to the same function.

This patch adds a test case for this situation and fixes the bug by
pruning identical functions in remove_extra_symbols.