Track LLVM 18 changes.

A function was given a newly standard name from C++20 in LLVM 16.  Then
LLVM 18 added a deprecation warning for the old name, and it is about to
ship, so it's time to adjust that.

Back-patch to all supported releases.

Discussion: https://www.postgresql.org/message-id/CA+hUKGLbuVhH6mqS8z+FwAn4=5dHs0bAWmEMZ3B+iYHWKC4-ZA@mail.gmail.com

src/backend/jit/llvm/llvmjit_inline.cpp

@@ -821,7 +821,10 @@ static void
 add_module_to_inline_search_path(InlineSearchPath& searchpath, llvm::StringRef modpath)
 {
 	/* only extension in libdir are candidates for inlining for now */
-	if (!modpath.startswith("$libdir/"))
+#if LLVM_VERSION_MAJOR < 16
+#define starts_with startswith
+#endif
+	if (!modpath.starts_with("$libdir/"))
 		return;
 
 	/* if there's no match, attempt to load */