[clang] [MinGW] Fix the regression caused by commit 592e935e115ffb451eb9b782376711dab6558fe0, that, in MinGW, Clang can't be built by system Clang 15.0.4. (PR #74982)

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

I don't know what issue/regression you're referring to. Please explain, in detail, what the issue is and all the relevant aspects of your configuration. Also explain what the suggested fix does, and how it handles the various cases (I just tested building latest llvm-project main with Clang 15 and LLD, for a mingw target, and it worked just fine, both as a regular non-dylib build, and with `LLVM_LINK_LLVM_DYLIB` enabled.) 

I also believe that the suggested patch would break actual use of clang-repl; if `LLVM_BUILD_LLVM_DYLIB` or `LLVM_BUILD_SHARED_LIBS` aren't defined, then those symbols wouldn't be dllexported at all. This causes them to not be found at runtime when the JIT runtime tries to locate them.

https://github.com/llvm/llvm-project/pull/74982