infcall: Add support for integer literals as reference function parameters

This patch attempts to mitigate the shortcomings of passing literals
to inferior function calls requiring references.  The specific use case here
is std::map's operator[]:

std::map int_map<int, int>;
int_map[1] = 10;
(gdb) print int_map[1]
Attempt to take address of value not located in memory.

This is occurring because while value_coerce_to_target understands
that some values need to be allocated and copied to the inferior's
memory, it only considers the actual parsed type of the argument value,
ignoring the actual type of the function parameter. That is,
in this specific case, the value's parsed type is TYPE_CODE_INT, but
the function requires TYPE_CODE_REF. We need to account for the
reference.

In value_arg_coerce, we have special handling for references, but it
has not specifically dealt with this case. It now checks if the
reference is in memory, and if it isn't, it copies it, if the type
is trivially copyable.

As a result of this patch, the last remaining failure in c++/15372 is now
fixed, and that bug can be closed.

With this patch, we can now print map entries with integer keys:

(gdb) print int_map[1]
$1 = (std::map<int, int, std::less<int>, std::allocator<std::pair<int const, int> > >::mapped_type &) @0x41f2d4: 10

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=15372
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=25957
Approved-By: Andrew Burgess <aburgess@redhat.com>