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.

`R_PPC64_GOT_TLSGD_PCREL34` is generated for pwr10+.

Note that llvm::support::endianness has been renamed to
llvm::endianness while becoming an enum class as opposed to an
enum. This patch replaces support::{big,little,native} with
llvm::endianness::{big,little,native}.

`R_PPC64_GOT_PCREL34` is generated for pwr10+.

Now that llvm::support::endianness has been renamed to
llvm::endianness, we can use the shorter form.  This patch replaces
support::endianness with llvm::endianness.

Now that llvm::support::endianness has been renamed to
llvm::endianness, we can use the shorter form.  This patch replaces
support::endianness::{big,little,native} with
llvm::endianness::{big,little,native}.

There is case that R_PPC64_REL24 with non-zero addend. The assertion is incorrectly triggered in such situation.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D158708

Reviewed By: sgraenitz

Differential Revision: https://reviews.llvm.org/D157226

RuntimeDyld has implemented more relocations than JITLink for ppc64. This patch adds relocations missing from JITLink for ppc64.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D157366