[ORC] Unify symbol mapping and allow custom mapping functions

[lucasreis1]

This PR provides four changes:

1. Unify symbol mapping into IRLayer and discard the old IRSymbolMapper
   class
2. Allow clients to provide their symbol mapper function to
   provide custom mappings
3. Skip adding linkonce_odr symbols to the symbol map on the default
   mapper (fixes [ORC] Missing definition for inlining function after optimizations in IRTRansformLayer  #64907)
4. Create a small FAQ section to explain to users why their LLVM IR symbol might be missing a definition from the JIT as a result of change 3.

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[github-actions]

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff 0f339e6567bffb290e409ef5de272fb75ce70234 5a2bbeb4c8b25d13da9aac729f40a4a5d4b491ec -- llvm/include/llvm/ExecutionEngine/Orc/Layer.h llvm/include/llvm/ExecutionEngine/Orc/Mangling.h llvm/lib/ExecutionEngine/Orc/CompileOnDemandLayer.cpp llvm/lib/ExecutionEngine/Orc/Layer.cpp llvm/lib/ExecutionEngine/Orc/Mangling.cpp

View the diff from clang-format here.

diff --git a/llvm/lib/ExecutionEngine/Orc/CompileOnDemandLayer.cpp b/llvm/lib/ExecutionEngine/Orc/CompileOnDemandLayer.cpp
index 007e19a8f87..73e927fd141 100644
--- a/llvm/lib/ExecutionEngine/Orc/CompileOnDemandLayer.cpp
+++ b/llvm/lib/ExecutionEngine/Orc/CompileOnDemandLayer.cpp
@@ -6,8 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "llvm/ADT/Hashing.h"
 #include "llvm/ExecutionEngine/Orc/CompileOnDemandLayer.h"
+#include "llvm/ADT/Hashing.h"
 #include "llvm/ExecutionEngine/Orc/ExecutionUtils.h"
 #include "llvm/IR/Mangler.h"
 #include "llvm/IR/Module.h"

[lucasreis1]

@lhames