This patch starts the initial implementation of JITLink for XCOFF (Object format for AIX).

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>

Identified with misc-include-cleaner.

This patch replaces uses of StringRef::{starts,ends}with with
StringRef::{starts,ends}_with for consistency with
std::{string,string_view}::{starts,ends}_with in C++20.

I'm planning to deprecate and eventually remove
StringRef::{starts,ends}with.

This allows these details to be shared with JITLink, which is allowed to
depend on the OrcShared library (but not on OrcJIT).

Based on the discussion in https://reviews.llvm.org/D130221 and https://reviews.llvm.org/D139223

Differential Revision: https://reviews.llvm.org/D139347

This patch mechanically replaces None with std::nullopt where the
compiler would warn if None were deprecated.  The intent is to reduce
the amount of manual work required in migrating from Optional to
std::optional.

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

Initial platform support for COFF/x86_64.

Completed features:
* Statically linked orc runtime.
* Full linking/initialization of static/dynamic vc runtimes and microsoft stl libraries.
* SEH exception handling.
* Full static initializers support
* dlfns
* JIT side symbol lookup/dispatch

Things to note:
* It uses vc runtime libraries found in vc toolchain installations.
* Bootstrapping state is separated because when statically linking orc runtime it needs microsoft stl functions to initialize the orc runtime, but static initializers need to be ran in order to fully initialize stl libraries.
* Process symbols can't be used blidnly on msvc platform; otherwise duplicate definition error gets generated. If process symbols are used, it's destined to get out-of-reach error at some point.
* Atexit currently not handled -- will be handled in the follow-up patches.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D130479

Properly set weak flag to COMDAT symbols so that no duplicate definition error will be generated. There is an inaccuracy in setting plain weak for largest selection type, which will be dealt with soon when largest type is properly implemented.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D129764