[llvm] get Linux `-fvisibility=hidden` shared library build working with GCC (#151365)

## Purpose
Add missing annotations so that LLVM can build as a shared library on
Linux with `-fvisibility=hidden` using GCC.

## Overview
Add a couple of annotations that make GCC happy:
* Add `LLVM_TEMPLATE_ABI` to the `extern` declaration of
`LoopBase<MachineBasicBlock, MachineLoop>`. The instantiation of this of
this template is already properly annotated with `LLVM_EXPORT_TEMPLATE`
and this declaration was missed. This omission did not cause problems
with MSVC or Clang but results in undefined reference to the destructor
with GCC.
* Add `LLVM_ATTRIBUTE_VISIBILITY_DEFAULT` to the template declaration
for `InnerAnalysisManagerProxy::Key`. `LLVM_ABI` cannot be used here
because MSVC disallows storage-class specifiers on class members outside
of the class declaration (C2720). Since
`LLVM_ATTRIBUTE_VISIBILITY_DEFAULT` only applies to non-Windows targets,
it can be used in its place. Omitting this annotation does not cause
problems with Clang but results in undefined reference to
`InnerAnalysisManagerProxy::Key` fields for for explicitly instantiated
declarations when compiling with GCC.

## 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).

## Validation
On Fedora 42, build with GCC `LLVM_BUILD_LLVM_DYLIB=ON`,
`LLVM_BUILD_LLVM_DYLIB_VIS=ON`, and `LLVM_LINK_LLVM_DYLIB=ON`.
```
cmake -B build -S llvm -G Ninja -DLLVM_ENABLE_PROJECTS="llvm;clang;clang-tools-extra;lldb;lld" -DLLVM_OPTIMIZED_TABLEGEN=ON -DLLVM_BUILD_TESTS=ON -DLLVM_BUILD_EXAMPLES=ON -DLLDB_ENABLE_PYTHON=NO -DLLVM_BUILD_LLVM_DYLIB=ON -DLLVM_BUILD_LLVM_DYLIB_VIS=ON -DLLVM_LINK_LLVM_DYLIB=ON -DCLANG_LINK_CLANG_DYLIB=OFF -DCMAKE_BUILD_TYPE=Release
ninja -C build
```

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