MFC r363013:

Merge commit 065fc1eafe7c from llvm git (by Richard Smith):

PR45521: Preserve the value kind when performing a standard
conversion sequence on a glvalue expression.

If the sequence is supposed to perform an lvalue-to-rvalue
conversion, then one will be specified as the first conversion in the
sequence. Otherwise, one should not be invented.

This should fix clang crashing with "can't implicitly cast lvalue to
rvalue with this cast kind", followed by "UNREACHABLE executed at
/usr/src/contrib/llvm-project/clang/lib/Sema/Sema.cpp:538!", when
building recent versions of Ceph, and the CPAN module SYBER/Date-5.2.0.

Reported by: Willem Jan Withagen <wjw@digiware.nl>, eserte12@yahoo.de
PR: 245530, 247812