This patch removes SMRange(std::nullopt_t) to reduce the number of
uses of std::nullopt outside the context of std::optional.  Since
there are only a handful of uses, this patch removes the constructor
without going through deprecation.

The use of std::nullopt here has its root in llvm::None, which was
used as a convenient way to indicate "nothing" before we migrated
llvm::Optional to std::optional.

* When `--enable-var-scope` is active,
`lib/FileCheck.cpp#clearLocalVars` gets called.
* That function loops through `GlobalNumericVariableTable` and then
calls `NumericVariable::clear` on most items. It also removes them from
`GlobalNumericVariableTable`.
* When reassigning an already cleared variable, `Pattern::match` calls
`NumericVariable::setValue`, but it doesn't reinsert it into
`GlobalNumericVariableTable`. Therefore, the next time `clearLocalVars`
is called, it won't be able to loop through the variables.

Fix it by reinserting them in `GlobalNumericVariableTable` inside
`Pattern::match`.

Co-authored-by: Thomas Preud'homme <thomas.preudhomme@arm.com>

## 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.

Firstly fix FileCheck printing string variables
double-escaped (first regex, then C-style).
    
This is confusing because it is not clear if the printed
value is the literal value or exactly how it is escaped, without
looking at FileCheck's source code.
    
Secondly, only escape when doing so makes it easier to read the value
(when the string contains tabs, newlines or non-printable characters).
When the variable value is escaped, make a note of it in the output too,
in order to avoid confusion.
    
The common case that is motivating this change is variables that contain
windows style paths with backslashes. These were printed as
`"C:\\\\Program Files\\\\MyApp\\\\file.txt"`.
Now prefer to print them as `"C:\Program Files\MyApp\file.txt"`.
Printing the value literally also makes it easier to search for
variables in the output, since the user can just copy-paste it.

These are identified by misc-include-cleaner.  I've filtered out those
that break builds.  Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.

We can use *Set::insert_range to collapse:

  for (auto Elem : Range)
    Set.insert(E);

down to:

  Set.insert_range(Range);

In some cases, we can further fold that into the set declaration.