The debug section map was using MachO section names (with the "__" prefix), but
DWARFContext expects section names with the object format prefix stripped off.
This was preventing DWARFContext from accessing the debug_str section,
resulting in bogus source name strings.

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 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.

Identified with clangd.

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}.

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}.

Without this patch, we pass G.getEndianness() as one of the parameters
to DWARFContext::create.  The problem is that G.getEndianness() is of:

  enum endianness {big, little, native};

whereas DWARFContext::create is expecting "bool isLittleEndian".  That
is, we are relying on an implicit conversion to convert big and little
to false and true, respectively.

When we migrate llvm::support::endianness to std::endian in future, we
can no longer rely on an implicit conversion because std::endian is
declared with "enum class".  Even if we could, the conversion would
not be guaranteed to work because, for example, libcxx defines:

  enum class endian {
    little = 0xDEAD,
    big = 0xFACE,
    :

where big and little are not boolean values.

This patch fixes the problem by properly converting G.getEndianness()
to a boolean value.

At line 191, `addSymbol` takes the name by reference but does not make
an internal copy to the string, meaning the local
`optional<std::string>` would get freed and leave Orc with a dangling
pointer. Fix this by just using an `optional<StringRef>` instead.

This re-applies db51e572893, which was reverted in 05b1a2cb3e6 due to bot
failures. The DebuggerSupportPlugin now depends on DWARF, so it has been moved
to the new OrcDebugging library (as has the enableDebuggerSupport API).