[c++-concepts] Merge from trunk

[Gabriel Dos Reis]

Trunk was merged to the c++-concept branch at revision 200958.

Andrew --
There aere a few conflicts surronding introduction of
can_convert_standard, between your version and the one Jason committed.
I resolved them in favour of Jason's because his version followed the
GNU coding convention (when a binary expression spans several lines, the
binary operator starts the new line, etc.)

The split of cp/semantics.c into cp/lambda.c and the rest introduced a
conflict at the Makefile level.  Resolved.

-- Gaby