YAMLIO: Improve template arg deduction for mapOptional

Summary:
The way c++ template argument deduction works, both arguments are used
to deduce the template type in the three-argument overload of
mapOptional. This is a problem if the types are slightly different, even
if they are implicitly convertible. This is fairly easy to trigger with
integral types, as the default type of most integral constants is int,
which then requires casting the constant to the type of the other
argument.

This patch fixes that by using a separate template type for the default
value, which is then cast to the type of the first argument.  To avoid
this conversion triggerring conversions marged as explicit, we use
static_assert to check that the types are implicitly convertible.

Reviewers: zturner, sammccall

Subscribers: kristina, jdoerfert, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59142

llvm-svn: 356157