Automerge: Reapply "[APINotes] Prefer diff -u over diff -b" (#157894)

This reverts commit 8d35bcc.

This was causing failures on MacOS due to the head command there not
supporting negative offsets. This patch fixes that by removing the call
to HEAD and relaxing the requirements around removing the last line of
the file.

Author: boomanaiden154

clang/test/APINotes/yaml-roundtrip-2.test

@@ -1,11 +1,9 @@
 RUN: apinotes-test %S/Inputs/Frameworks/SimpleKit.framework/Headers/SimpleKit.apinotes > %t.result
-RUN: not diff -b -e %t.result %S/Inputs/Frameworks/SimpleKit.framework/Headers/SimpleKit.apinotes | FileCheck %s
-
-The `-e` option of `diff` is not implemented in the builtin diff, assume
-that we have a POSIX compatible diff when we have a shell.
-REQUIRES: shell
+RUN: not diff -u %S/Inputs/Frameworks/SimpleKit.framework/Headers/SimpleKit.apinotes %t.result | \
+RUN:   tail -n +4 | \
+RUN:   FileCheck %s --implicit-check-not="{{^\+}}" --implicit-check-not="{{^\-}}"
 
 We expect only the document markers to be emitted
 
-CHECK: 52d
-CHECK: 1d
+CHECK: +---
+CHECK: +...

clang/test/APINotes/yaml-roundtrip.test

@@ -1,30 +1,23 @@
 RUN: apinotes-test %S/Inputs/Frameworks/Simple.framework/Headers/Simple.apinotes > %t.result
-RUN: not diff -b %S/Inputs/Frameworks/Simple.framework/Headers/Simple.apinotes %t.result | FileCheck %s
-
-Avoid Windows as the diff output differs due to line-endings and different diff
-implementations.
-UNSUPPORTED: system-windows
+RUN: not diff -u %S/Inputs/Frameworks/Simple.framework/Headers/Simple.apinotes %t.result | \
+RUN:   tail -n +5 | \
+RUN:   FileCheck %s --implicit-check-not="{{^\+}}" --implicit-check-not="{{^\-}}"
 
 We expect only the nullability to be different as it is canonicalized during the
 roudtrip.
 
-CHECK:      7c8
-CHECK-NEXT: <         Nullability:     N
-CHECK-NEXT: ---
-CHECK-NEXT: >         Nullability:     Nonnull
-CHECK-NEXT: 13c14
-CHECK-NEXT: <         Nullability:     O
-CHECK-NEXT: ---
-CHECK-NEXT: >         Nullability:     Optional
-CHECK-NEXT: 19c20
-CHECK-NEXT: <         Nullability:     U
-CHECK-NEXT: ---
-CHECK-NEXT: >         Nullability:     Unspecified
-CHECK-NEXT: 25c26
-CHECK-NEXT: <         Nullability:     S
-CHECK-NEXT: ---
-CHECK-NEXT: >         Nullability:     Unspecified
-CHECK-NEXT: 28c29
-CHECK-NEXT: <         Nullability:     Scalar
-CHECK-NEXT: ---
-CHECK-NEXT: >         Nullability:     Unspecified
+CHECK:      -        Nullability:     N
+CHECK-NEXT: +        Nullability:     Nonnull
+CHECK:      -        Nullability:     O
+CHECK-NEXT: +        Nullability:     Optional
+CHECK:      -        Nullability:     U
+CHECK-NEXT: +        Nullability:     Unspecified
+CHECK:      -        Nullability:     S
+CHECK-NEXT: +        Nullability:     Unspecified
+CHECK:      -        Nullability:     Scalar
+CHECK-NEXT: +        Nullability:     Unspecified
+
+# The roundtrip will add document markers. It is hard to remove the last line of the
+# file in a cross-platform manner, so just assert it here to avoid a test failure due
+# to the implicit check not.
+# CHECK: +