[LTO][CodeGen] Test LTO pipeline choices more thoroughly (llvm#65485)

The test was looking only for the bitcode writer pass; it should check
the full pipeline to make sure the choice is as intended.

Author: pogo59

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

@@ -1,12 +1,18 @@
-// RUN: %clang --target=x86_64-unknown-unknown-linux -Xclang -fdebug-pass-manager -flto=thin -funified-lto -O2 -c %s -o %t.0 2>&1 | FileCheck --check-prefix=THIN %s
-// RUN: mv %t.0 %t.1
-// RUN: %clang --target=x86_64-unknown-unknown-linux -Xclang -fdebug-pass-manager -flto=full -funified-lto -O2 -c %s -o %t.0 2>&1 | FileCheck --check-prefix=THIN %s
-// RUN: %clang --target=x86_64-unknown-unknown-linux -Xclang -fdebug-pass-manager -flto=thin -O2 -c %s -o %t.2 2>&1 | FileCheck --check-prefix=THIN %s
-// RUN: mv %t.2 %t.3
-// RUN: %clang --target=x86_64-unknown-unknown-linux -Xclang -fdebug-pass-manager -flto=full -O2 -c %s -o %t.2 2>&1 | FileCheck --check-prefix=FULL %s
-// RUN: cmp %t.0 %t.1
+// RUN: %clang --target=x86_64-unknown-unknown-linux -Xclang -fdebug-pass-manager -flto=thin -funified-lto -O2 -c %s -o %t.0 2>%t.0.txt
+// RUN: %clang --target=x86_64-unknown-unknown-linux -Xclang -fdebug-pass-manager -flto=full -funified-lto -O2 -c %s -o %t.1 2>%t.1.txt
+// RUN: %clang --target=x86_64-unknown-unknown-linux -Xclang -fdebug-pass-manager -flto=thin -O2 -c %s -o %t.2 2>%t.2.txt
+// RUN: %clang --target=x86_64-unknown-unknown-linux -Xclang -fdebug-pass-manager -flto=full -O2 -c %s -o %t.3 2>%t.3.txt
+// RUN: FileCheck --input-file %t.0.txt %s --check-prefix=THIN
+// RUN: FileCheck --input-file %t.3.txt %s --check-prefix=FULL
 // THIN: ThinLTOBitcodeWriterPass
 // FULL-NOT: ThinLTOBitcodeWriterPass
+/// Check that thin/full unified bitcode matches.
+// RUN: cmp %t.0 %t.1
+/// 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
 
 int foo() {
   return 2 + 2;