Enable ELFNixPlatform support for loongarch64. These are few simple
changes, but it allows us to use the orc runtime in ELF/LoongArch64
backend.

This change adds test cases targeting the LoongArch64 Linux platform to
the ORC runtime integration test suite. Since jitlink for loongarch64 is
ready for general use, and ELF-based platforms support defining multiple
static initializer table sections with differing priorities, some
relevant test cases in compiler-rt for ELFNixPlatform support can be
enabled.

Removes LinkGraph's PointerSize and Endianness members and uses the triple to
find these values instead.

Also removes some redundant Triple copies.

This re-applies 570ecdcf8b4, which was reverted in 74e8a37ff32 due to bot
failures. This commit renames sysv_resolve.cpp to resolve.cpp, which was the
cause of the config errors.

This reverts commit 41652c6c92958a87b8505b9b1e6f008856e392ac while I investigate
more bot failures.

This reapplies 570ecdcf8b4, which was reverted in 6073dd923b8 due to bot
failures.

The test failures on Linux were fixed by:
1. Removing an overly restrictive assertion (query dependence on a symbol no
longer implies a MaterializingInfo for that symbol)
2. Adding reentry and resolver files to the ORC runtime CMakeLists.txt for
Linux.
3. Adding the __orc_rt_reentry -> __orc_rt_sysv_reentry alias to ELFNixPlatform.

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)