Introduces a new layer interface, LinkGraphLayer, that can be used to
add LinkGraphs to an ExecutionSession.

This patch moves most of ObjectLinkingLayer's functionality into a new
LinkGraphLinkingLayer which should (in the future) be able to be used
without linking libObject. ObjectLinkingLayer now inherits from
LinkGraphLinkingLayer and just handles conversion of object files to
LinkGraphs, which are then handed down to LinkGraphLinkingLayer to be
linked.

This class is an implementation detail, so doesn't need a publicly accessible
name.

Use SymbolStringPtr for Symbol names in LinkGraph. This reduces string interning
on the boundary between JITLink and ORC, and allows pointer comparisons (rather
than string comparisons) between Symbol names. This should improve the
performance and readability of code that bridges between JITLink and ORC (e.g.
ObjectLinkingLayer and ObjectLinkingLayer::Plugins).

To enable use of SymbolStringPtr a std::shared_ptr<SymbolStringPool> is added to
LinkGraph and threaded through to its construction sites in LLVM and Bolt. All
LinkGraphs that are to have symbol names compared by pointer equality must point
to the same SymbolStringPool instance, which in ORC sessions should be the pool
attached to the ExecutionSession.
---------

Co-authored-by: Lang Hames <lhames@gmail.com>

This reapplies aba6bb0820b, which was reverted in 28e2a891210 due to bot
failures. It contains fixes to silence warnings for uncovered switches,
and for incorrect initializer-symbol handling on ELF and COFF.

This reverts commit aba6bb0820b247d4caf4b5e00810909214a58053 while I investigate bot
failures (e.g. https://lab.llvm.org/buildbot/#/builders/143/builds/3848)

SideEffectsOnly is a new jitlink::Scope value that corresponds to the
JITSymbolFlags::MaterializationSideEffectsOnly flag: Symbols with this scope
can be looked up (and form part of the initial interface of a LinkGraph) but
never actually resolve to an address (so can only be looked up with a
WeaklyReferencedSymbol lookup).

Previously ObjectLinkingLayer implicitly treated JITLink symbols as having this
scope, regardless of a Symbol's actual scope, if the
MaterializationSideEffectsOnly flag was set on the corresponding symbol in the
MaterializationResponsibility object. Using an explicit scope in JITLink for
this (1) allows JITLink plugins to identify and correctly handle
side-effects-only symbols, and (2) allows raw LinkGraphs to define
side-effects-only symbols without clients having to manually modify their
`MaterializationUnit::Interface`.

Non-locally scoped symbols must have names.

Identified with misc-include-cleaner.

When transferring resources, the destination tracker key may not be in
the internal map, invalidating iterators and value references. The added
test creates such situation and would fail before with "Finalized
allocation was not deallocated."

For good measure, fix the same pattern in RTDyldObjectLinkingLayer
which is harder to test because it "only" results in memory managers
being deleted in the wrong order.