Do export symbols when LLVM_EXPORT_SYMBOLS_FOR_PLUGINS is on.

Do export symbols when LLVM_EXPORT_SYMBOLS_FOR_PLUGINS is on.

Summary:
In D76527, we stopped exporting symbols from clang, opt and llc unless
the `LLVM_ENABLE_PLUGINS` cmake variable is true (which causes clang's
own plugin collection to be built).

But another reasonable build configuration is to ask clang to export
its symbols for out-of-tree plugins to use, without building the
in-tree ones. That is, you might set `LLVM_EXPORT_SYMBOLS_FOR_PLUGINS`
without also setting `LLVM_ENABLE_PLUGINS` (at least if you're using
MSVC, where you need to ask explicitly for the symbols to be
exported).

In that situation, the symbols should still be exported, but after
D76527, they weren't being.

Reviewers: efriedma, john.brawn

Reviewed By: efriedma, john.brawn

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76760