Improve ebuild for .NET Core (LLVM-4 and heimdal)

[dmitryvk]

This adds support for building .NET Core with LLVM 4 (mostly backported
from dotnet/coreclr#11226).
Also added a 'heimdal' USE flag to allow use of Heimdal implementation
of Kerberos rather than MIT-KRB5.

[dmitryvk]

Note that the dependency specification is somewhat wrong: I've added >= llvm-4.0 and >= lldb-4.0 because llvm-4 does not use lldb use flag anymore; it would be better to have something like (>= llvm-4.0 and >= lldb-4.0) OR (>=llvm-3.7.1[lldb]) but I couldn't find the right syntax for it.
Also I didn't want to add heimdal USE flag and instead depend on virtual/krb5; but I didn't figure out how to determine which package was chosen by virtual package.