[LTO] Enhance unified/nonunified LTO checks. (llvm#148229)

For the PS targets, unified LTO pipeline is used as default behaviour when enabling LTO. Other targets use Distinct LTO pipeline behaviour as default behavious. Unified/nonunified LTO checks are enhanced in this PR:
1. Check that the default, unified, and non-unified behavior for asan-unified-lto.ll all match (irrespective of what the default unified/nonunified behavior is).
2. Check that the difference in behavior for 'unified' and 'full' runs is (regarding the passes) is what is expected (rather than simply that they are different).

Author: MaggieYingYi

clang/test/CodeGen/asan-unified-lto.ll

@@ -5,6 +5,7 @@
 
 ; RUN: %clang_cc1 -emit-llvm-bc -O1 -flto -fsanitize=address -o - -x ir < %s | llvm-dis -o - | FileCheck %s
 ; RUN: %clang_cc1 -emit-llvm-bc -O1 -flto -funified-lto -fsanitize=address -o - -x ir < %s | llvm-dis -o - | FileCheck %s
+; RUN: %clang_cc1 -emit-llvm-bc -O1 -flto -fno-unified-lto -fsanitize=address -o - -x ir < %s | llvm-dis -o - | FileCheck %s
 ; CHECK: @anon.3ee0898e5200a57350fed5485ae5d237
 
 target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"

clang/test/CodeGen/unified-lto-pipeline.c

@@ -11,8 +11,10 @@
 /// Check that pass pipelines for thin, thin-unified, full-unified all match.
 // RUN: diff %t.0.txt %t.1.txt
 // RUN: diff %t.0.txt %t.2.txt
-/// Pass pipeline for full is different.
-// RUN: not diff %t.0.txt %t.3.txt
+/// Pass pipeline for full is different. Unified uses the full Linux pipeline except ThinLTOBitcodeWriterPass vs BitcodeWriterPass.
+// RUN: not diff -u %t.0.txt %t.3.txt | FileCheck %s --check-prefix=DIFF --implicit-check-not="{{^[-+!<>] }}"
+// DIFF:      -Running pass: ThinLTOBitcodeWriterPass
+// DIFF-NEXT: +Running pass: BitcodeWriterPass
 
 int foo() {
   return 2 + 2;