Collapse the three separate initialization paths in

TryStaticImplicitCast (for references, class types, and everything
else, respectively) into a single invocation of
InitializationSequence.

One of the paths (for class types) was the only client of
Sema::TryInitializationByConstructor, which I have eliminated. This
also simplified the interface for much of the cast-checking logic,
eliminating yet more code.

I've kept the representation of C++ functional casts with <> 1
arguments the same, despite the fact that I hate it. That fix will
come soon. To satisfy my paranoia, I've bootstrapped + tested Clang
with these changes.

llvm-svn: 101549