[flang] Fix copy elision assumption.

Before this patch, the Restorer depended on copy elision to happen.
Without copy elision, the function ScopedSet calls the move constructor
before its dtor. The dtor will prematurely restore the reference to the
original value.

Instead of relying the compiler to not use the Restorer's copy
constructor, delete its copy and assign operators. Hence, callers cannot
move or copy a Restorer object anymore, and have to explicitly provide
the reset state. ScopedSet avoids calling move/copy operations by
relying on unnamed return value optimization, which is mandatory in
C++17.

Reviewed By: klausler

Differential Revision: https://reviews.llvm.org/D88797