x86_64::GOTTableManager and x86_64::PLTTableManager will now look for existing
GOT and PLT sections and re-use existing entries if they're present.

This will be used for an upcoming MachO patch to enable compact unwind support.

This patch is the x86-64 counterpart 42595bdaefb, which added the same
functionality to the GOT and PLT managers for aarch64.

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 patch adds support for R_X86_64_SIZE32/R_X86_64_SIZE64 relocation
types by introducing edge kinds x86_64::Size32/x86_64::Size64. The
calculation for these relocations is: Z + A, where:

Z - Represents the size of the symbol whose index resides in the
    relocation entry.

A - Represents the addend used to compute the value of the relocation
    field.

Ref: [System V Application Binary Interface
x86-64](https://gitlab.com/x86-psABIs/x86-64-ABI/-/jobs/artifacts/master/raw/x86-64-ABI/abi.pdf?job=build)

This patch adds support for R_X86_64_PC16 relocation type and
x86_64::Delta16 edge kind. This patch also adds missing test cases for
R_X86_64_PC32, R_X86_64_PC64 relocation types.

Add support for ELF::R_X86_64_PC8 relocation via new x86_64::Delta8 edge
kind.

This commit adds section start and stop symbol handling to ELF/aarch64, and
fixes the section symbol prefixes (using `__start_` and `__stop_`, rather than
`__start` and `__end`). It also adds a testcase for handling of these symbols.

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.

These files satisfy all of the following:

- misc-include-cleaner indicates that these files do not need
  Endian.h.
- They do not mention "endian" anywhere.
- They do not include any *.inc or *.def, which could need
  llvm::support::endian.

The LinkGraph that is passed into the ELFJITLinker_x86_64 constructor is handed
off to the JITLinker base class, so can't be used in the constructor body. This
commit adds a getGraph method to JITLinker, and updates ELFJITLinker_x86_64 to
use that.

This allows JITLinkerBase implementations to check whether default passes
should be added. The ELF_x86_64 backend is updated to check this predicate
before installing the getOrCreateGOTSymbol pass.

This is an alternative solution to https://reviews.llvm.org/D158909.