[clang] [flang] [llvm] [polly] Import and Export `getXXXPluginInfo` from embedded plugin such as Polly (PR #156440)

[Tomohiro Kashiwada via llvm-commits]

kikairoya wrote:

> The export side of this looks like a CMake bug: if we're linking code into the LLVM dylib, add_llvm_pass_plugin() should configure LLVM_ABI so it exports any marked functions.

Having reconsidered this, `DISABLE_LLVM_LINK_LLVM_DYLIB` seems to behave more like `-Wl,-Bstatic`, i.e. "this is built on top of libLLVM; link it statically," rather than "this is part of libLLVM."  
I think we need a new option for `add_llvm_library` to handle this case explicitly, named something like `MERGE_INTO_LLVM_DYLIB`.

I've posted a new PR: https://github.com/llvm/llvm-project/pull/158023

https://github.com/llvm/llvm-project/pull/156440