When JITLink reports an out-of-range error, the underlying reason could
be hidden from the user if it's due to an excessively large target
addend. Add non-zero target addend to the message for clarity.

When building on i386, both clang and gcc define a builtin 'i386' macro (see
discussion in https://github.com/llvm/llvm-project/pull/137063). This causes
build errors in the JITLink/i386 backend when attempting to build LLVM on i386.

This commit renames the 'i386' backend (namespaces, APIs and files) to 'x86' to
avoid this issue.

This patch replaces:

  llvm::copy(Src, std::back_inserter(Dst));

with:

  llvm::append_range(Dst, Src);

for breavity.

One side benefit is that llvm::append_range eventually calls
llvm::SmallVector::reserve if Dst is of llvm::SmallVector.

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

`Sym.getSection()` is equivalent to `Sym.getBlock().getSection()`.

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

Also removes some redundant Triple copies.

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.

Symbol::setSize asserts that the new size does not overflow the containing
block, so we need to point the Symbol at the correct Block before updating its
size (otherwise we may get a spurious overflow assertion).