The bcache uses memcpy to make copies of the data passed to it.  In
C++, this is only safe for trivially-copyable types.

This patch changes bcache to require this property, and slightly
changes the API to make it easier to use when copying a single object.
It also makes the new 'insert' template methods return the correct
type.
---
 gdb/bcache.h   | 25 ++++++++++++++++++++++++-
 gdb/gdbtypes.c |  2 +-
 gdb/macrotab.c | 12 ++++++------
 gdb/psymtab.c  |  6 ++----
 4 files changed, 33 insertions(+), 12 deletions(-)