[c++1z] P0012R1: Implement a few remaining pieces: downgrade diagnostic for

mismatched dynamic exception specifications in expressions from an error to a
warning, since this is no longer ill-formed in C++1z.

Allow reference binding of a reference-to-non-noexcept function to a noexcept
function lvalue. As defect resolutions, also allow a conditional between
noexcept and non-noexcept function lvalues to produce a non-noexcept function
lvalue (rather than decaying to a function pointer), and allow function
template argument deduction to deduce a reference to non-noexcept function when
binding to a noexcept function type.

llvm-svn: 284905