[LLVM]Fix symbol visibility macros not being define for AIX (#107705)

This is to try and fix buildbot failure on
[clang-ppc64-aix](https://lab.llvm.org/buildbot/#/builders/64/builds/881)
from my changes in #96630. I don't really know much about AIX so it
would be good to have someone more knowledgeable to say if visibility
macros on extern templates is needed on AIX similar to ELF. @compnerd
@tstellar

Author: fsfod

llvm/include/llvm/Support/Compiler.h

@@ -189,7 +189,7 @@
 #define LLVM_TEMPLATE_ABI __declspec(dllimport)
 #define LLVM_EXPORT_TEMPLATE
 #endif
-#elif defined(__ELF__) || defined(__MINGW32__)
+#elif defined(__ELF__) || defined(__MINGW32__) || defined(_AIX)
 #define LLVM_ABI LLVM_ATTRIBUTE_VISIBILITY_DEFAULT
 #define LLVM_TEMPLATE_ABI LLVM_ATTRIBUTE_VISIBILITY_DEFAULT
 #define LLVM_EXPORT_TEMPLATE