[CAS] Fix AIX build (llvm#159647)

Fix AIX build by linking `libbsd.a` to include implementation for
`flock`.

Author: cachemeifyoucan

llvm/lib/CAS/CMakeLists.txt

@@ -1,3 +1,7 @@
+if (UNIX AND "${CMAKE_SYSTEM_NAME}" MATCHES "AIX")
+  set(additional_libs bsd)
+endif()
+
 add_llvm_component_library(LLVMCAS
   ActionCache.cpp
   ActionCaches.cpp
@@ -20,6 +24,7 @@ add_llvm_component_library(LLVMCAS
 
   LINK_LIBS
   ${LLVM_PTHREAD_LIB}
+  ${additional_libs}
 
   LINK_COMPONENTS
   Support