## Purpose
Export a small number of private LLVM symbols so that unit tests can
still build/run when LLVM is built as a Windows DLL or a shared library
with default hidden symbol visibility.

## Background
The effort to build LLVM as a WIndows DLL is tracked in #109483.
Additional context is provided in [this
discourse](https://discourse.llvm.org/t/psa-annotating-llvm-public-interface/85307).

Some LLVM unit tests use internal/private symbols that are not part of
LLVM's public interface. When building LLVM as a DLL or shared library
with default hidden symbol visibility, the symbols are not available
when the unit test links against the DLL or shared library.

This problem can be solved in one of two ways:
1. Export the private symbols from the DLL.
2. Link the unit tests against the intermediate static libraries instead
of the final LLVM DLL.

This PR applies option 1. Based on the discussion of option 2 in
#145448, this option is preferable.

## Overview
* Adds a new `LLVM_ABI_FOR_TEST` export macro, which is currently just
an alias for `LLVM_ABI`.
* Annotates the sub-set of symbols under `llvm/lib` that are required to
get unit tests building using the new macro.

This changes the final stage of InstrRef, i.e. the TransferTracker
(which combines the values locations with the variable values), so that
it treats a DEBUG_VALUE of an EntryValue just like a DEBUG_VALUE of a
constant: a location that is never clobbered and can be propagated to
subsequent BBs as long as no other DEBUG_VALUE intrinsics updated the
variable.

We add two tests here:

1. `entry_value_clobbered_stack_copy` that saves a register on the
stack, uses this register as an entry value DBG_VALUE location, and then
clobbers it. Prior to this patch, this test would crash because we would
try to describe a new location for the variable in terms of what was
saved on the stack, and use an invalid expression to do so. This is not
needed as an EntryValue can never be clobbered.

2. `entry_value_gets_propagated`, that tests that an EntryValue
DBG_VALUE is propagated in a diamond-shaped CFG.

This patch is trying to reland
https://github.com/llvm/llvm-project/pull/77938 but also fixes the bug
with InstrRef based LiveDebugValues, where entry values were not being
propagated in a diamond-shaped CFG.

try_emplace can default-construct values, so we do not need to do so
on our own.  Plus, try_emplace(Key) is much simpler/shorter than
insert({Key, LongValueType()}).

This is a reland of #138434 except that:

- the bits for llvm/lib/CodeGen/RenameIndependentSubregs.cpp
  have been dropped because they caused a test failure under asan, and

- the bits for llvm/lib/CodeGen/SelectionDAG/ScheduleDAGFast.cpp have
  been improved with structured bindings.

This reverts commit a9699a334bc9666570418a3bed9520bcdc21518b.

Breaks CodeGen/AMDGPU/collapse-endcf.ll in several configs
(sanitizer builds; macOS; possibly more), see comments on
https://github.com/llvm/llvm-project/pull/138434