[clangd] Fix building SerializationTests unit test on OpenBSD

This fixes building the unit tests on OpenBSD. OpenBSD does not support RLIMIT_AS.

Reviewed By: kadircet

Differential Revision: https://reviews.llvm.org/D119989

(cherry picked from commit f374c8d)

clang-tools-extra/clangd/unittests/SerializationTests.cpp

@@ -308,9 +308,9 @@ TEST(SerializationTest, CmdlTest) {
   }
 }
 
-// rlimit is part of POSIX.
+// rlimit is part of POSIX. RLIMIT_AS does not exist in OpenBSD.
 // Sanitizers use a lot of address space, so we can't apply strict limits.
-#if LLVM_ON_UNIX && !LLVM_ADDRESS_SANITIZER_BUILD &&                           \
+#if LLVM_ON_UNIX && defined(RLIMIT_AS) && !LLVM_ADDRESS_SANITIZER_BUILD &&     \
     !LLVM_MEMORY_SANITIZER_BUILD
 class ScopedMemoryLimit {
   struct rlimit OriginalLimit;