[llvm] get cl::opt instantiations working with MSVC DLL build (#147810)

## Purpose

This patch is one in a series of code-mods that annotate LLVM’s public
interface for export. This patch annotates the `llvm::cl::opt` explicit
template instantiations for export with `LLVM_TEMPLATE_ABI` and
`LLVM_EXPORT_TEMPLATE`. This annotation currently has no meaningful
impact on the LLVM build; however, it is a prerequisite to support an
LLVM Windows DLL (shared library) build.

## Background

This effort is tracked in #109483. Additional context is provided in
[this
discourse](https://discourse.llvm.org/t/psa-annotating-llvm-public-interface/85307),
and documentation for `LLVM_ABI` and related annotations is found in the
LLVM repo
[here](https://github.com/llvm/llvm-project/blob/main/llvm/docs/InterfaceExportAnnotations.rst).

Annotating the `llvm::cl::opt` template instances for DLL export was not
straight-forward like other explicit template instances that have
already been annotated. Annotating them as documented
[here](https://github.com/llvm/llvm-project/blob/main/llvm/docs/InterfaceExportAnnotations.rst#templates)
results in link errors when building a Windows DLL using MSVC.

## Overview 
There are two specific issues that appear when exporting the
`llvm::cl::opt` templates and compiling a Windows DLL with MSVC:
1. We cannot export `opt<std::string>`. This is because MSVC exports all
ancestor classes when exporting an instantiated template class. Since
one of `opt`'s ancestor classes is its type argument (via
`opt_storage`), it is an ancestor of `std::string`. Therefore, if we
export `opt<std::string>` from the LLVM DLL, MSVC forces
`std::basic_string` to also be exported. This leads to duplicate symbol
errors and generally seems like a bad idea. Compiling with `clang-cl`
does not exhibit this behavior.
2. The `opt` template instances other than `opt<bool>` get optimized out
because they are not referenced in the TU (`opt<bool>` actually is). It
is unclear exactly why MSVC optimizes these template instances away, but
`clang-cl` does not. Adding explicit references to the instantiated
`opt` template classes' vtables via implicit virtual destructor forces
MSVC to export them.

## Validation
Windows with MSVC
Windows with Clang

1 files changed, 22 insertions, 5 deletions

diff --git a/llvm/lib/Support/CommandLine.cpp b/llvm/lib/Support/CommandLine.cpp
index d5c3cba..8491633 100644
--- a/llvm/lib/Support/CommandLine.cpp
+++ b/llvm/lib/Support/CommandLine.cpp
@@ -68,11 +68,19 @@ template class LLVM_EXPORT_TEMPLATE basic_parser<float>;
 template class LLVM_EXPORT_TEMPLATE basic_parser<std::string>;
 template class LLVM_EXPORT_TEMPLATE basic_parser<char>;
 
-template class opt<unsigned>;
-template class opt<int>;
-template class opt<std::string>;
-template class opt<char>;
-template class opt<bool>;
+#if !(defined(LLVM_ENABLE_LLVM_EXPORT_ANNOTATIONS) && defined(_MSC_VER))
+// Only instantiate opt<std::string> when not building a Windows DLL. When
+// exporting opt<std::string>, MSVC implicitly exports symbols for
+// std::basic_string through transitive inheritance via std::string. These
+// symbols may appear in clients, leading to duplicate symbol conflicts.
+template class LLVM_EXPORT_TEMPLATE opt<std::string>;
+#endif
+
+template class LLVM_EXPORT_TEMPLATE opt<bool>;
+template class LLVM_EXPORT_TEMPLATE opt<char>;
+template class LLVM_EXPORT_TEMPLATE opt<int>;
+template class LLVM_EXPORT_TEMPLATE opt<unsigned>;
+
 } // namespace cl
 } // namespace llvm
 
@@ -95,6 +103,15 @@ void parser<float>::anchor() {}
 void parser<std::string>::anchor() {}
 void parser<char>::anchor() {}
 
+// These anchor functions instantiate opt<T> and reference its virtual
+// destructor to ensure MSVC exports the corresponding vtable and typeinfo when
+// building a Windows DLL. Without an explicit reference, MSVC may omit the
+// instantiation at link time even if it is marked DLL-export.
+void opt_bool_anchor() { opt<bool> anchor{""}; }
+void opt_char_anchor() { opt<char> anchor{""}; }
+void opt_int_anchor() { opt<int> anchor{""}; }
+void opt_unsigned_anchor() { opt<unsigned> anchor{""}; }
+
 //===----------------------------------------------------------------------===//
 
 const static size_t DefaultPad = 2;