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.

Identified with misc-include-cleaner.

Adds a function to create a LinkGraph of absolute symbols, and a
callback in dynamic library search generators to enable using it to
expose its symbols to the platform/orc runtime. This allows e.g. using
__orc_rt_run_program to run a precompiled function that was found via
dlsym. Ideally we would use this in llvm-jitlink's own search generator,
but it will require more work to align with the Process/Platform
JITDylib split, so not handled here.

As part of this change we need to handle LinkGraphs that only have
absolute symbols.

If JITLinkGeneric::linkPhase2 receives an Error rather than an InFlightAlloc
then we need to call JITLinkContext::notifyFailed, rather than calling
abandonAllocAndBailOut -- the latter asserts that there is an allocation to
abandon, and this was turning allocation errors into assertion failures in
debug mode.

ExecutorAddr was introduced in b8e5f918166 as an eventual replacement for
JITTargetAddress. ExecutorSymbolDef is introduced in this patch as a
replacement for JITEvaluatedSymbol: ExecutorSymbolDef is an (ExecutorAddr,
JITSymbolFlags) pair, where JITEvaluatedSymbol was a (JITTargetAddress,
JITSymbolFlags) pair.

A number of APIs had already migrated from JITTargetAddress to ExecutorAddr,
but many of ORC's internals were still using the older type. This patch aims
to address that.

Some public APIs are affected as well. If you need to migrate your APIs you can
use the following operations:

* ExecutorAddr::toPtr replaces jitTargetAddressToPointer and
  jitTargetAddressToFunction.

* ExecutorAddr::fromPtr replace pointerToJITTargetAddress.

* ExecutorAddr(JITTargetAddress) creates an ExecutorAddr value from a
  JITTargetAddress.

* ExecutorAddr::getValue() creates a JITTargetAddress value from an
  ExecutorAddr.

JITTargetAddress and JITEvaluatedSymbol will remain in JITSymbol.h for now, but
the aim will be to eventually deprecate and remove these types (probably when
MCJIT and RuntimeDyld are deprecated).

If an error occurs during the post-allocation phase (JITLinkerBase::linkPhase2)
then we need to call JITLinkMemoryManager::InFlightAlloc::abandon so that the
allocation has a chance to clean up. This was already handled for later phases,
but we were skipping the abandon step when we bailed out of phase 2.

This is a counterpart to ffe2dda29f3, and does for scope what that commit did
for linkage.

Making the scope of external definitions visible to JITLink plugins will
allow us to distinguish hidden weak defs (which do not need to be tracked by
default) from default-scoped weak defs (which need to be updated to point at
a single chosen definition at runtime).

Previously we stripped Weak flags from JITDylib symbol table entries once they
were resolved (there was no particularly good reason for this). Now we want to
retain them and query them when setting the Linkage on external symbols in
LinkGraphs during symbol resolution (this was the motivation for 75404e9ef88).
Making weak linkage of external definitions discoverable in the LinkGraph will
in turn allow future plugins to implement correct handling for them (by
recording locations that depend on exported weak definitions and pointing all
of these at one chosen definition at runtime).

Introduces two new methods on Symbol: isWeaklyReferenced and
setWeaklyReferenced. These are now used to track/set whether an external symbol
is weakly referenced, rather than having the Symbol's linkage set to weak.

This change is a first step towards proper handling of weak defs used across
JITDylib boundaries: It frees up the Linkage field on external symbols so that
it can be used to represent the linkage of the definition that the symbol resolves
to. It is expected that Platform plugins will use this information to track
locations that need to be updated if the selected weak definition changes (e.g.
because JITDylibs were dlclosed and then dlopened again in a different order).