Adds new convenience methods findDefinedSymbolByName, findExternalSymbolByName
and findAbsoluteSymbolByName to the LinkGraph class. These should be used to
find symbols of the given types by name.

COFFLinkGraphBuilder and MachOPlatform are updated to take advantage of the
new methods.

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

Also removes some redundant Triple copies.

This fixes a concurrency bug in the COFF_x86_64 backend: lookupAsync was used
to find __ImageBase without blocking to wait for the result.

Rather than adding promises / futures, this patch just adds an external
__ImageBase symbol if needed. This is the canonical JITLink solution for
resolving external symbols, and is simpler and more concurrency friendly than
using promises / futures with lookupAsync.

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 785d376d123, which was reverted in c49837f5f68 due to bot
failures. The fix was to relax some asserts to allow common symbols to be
resolved with either common or weak flags, rather than requiring one or the
other.

This reverts commit 785d376d1231167688dd12f93c5c0a5d46cd4086 while I
investigate some bot failures (e.g.
https://lab.llvm.org/buildbot/#/builders/3/builds/1983).

Duplicate common definitions should be coaleseced, rather than being treated as
duplicate definitions. Strong definitions should override common definitions.

rdar://132314264

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 with llvm::endianness.