all: add LLVM 12 support

Originally based on a PR by @QuLogic, but extended a lot to get all
tests to pass.

transform/transform_test.go

@@ -116,8 +116,9 @@ func filterIrrelevantIRLines(lines []string) []string {
 		if strings.HasPrefix(line, "source_filename = ") {
 			continue
 		}
-		if llvmVersion < 11 && strings.HasPrefix(line, "attributes ") {
+		if llvmVersion < 12 && strings.HasPrefix(line, "attributes ") {
 			// Ignore attribute groups. These may change between LLVM versions.
+			// Right now test outputs are for LLVM 12.
 			continue
 		}
 		out = append(out, line)