[Utils] Ensure that empty.txt is always empty (llvm#157576)

Previously this test case would `touch %S/empty.txt` to create and empty
file. The test case then copies contents to that file, so if run a
second time the `touch` command would not create an empty file.

`llvm/utils/lit/tests/diff-test-update.py` also no longer matches against file
paths other than the final bit of the file name. This prevents test failures
on Windows due to different path separators.

(cherry picked from commit 82ef4ee)

Author: hnrklssn

llvm/utils/lit/tests/Inputs/diff-test-update/.gitignore

@@ -1 +1,2 @@
-*.txt
+; diff-tmp-dir.test clobbers this file
+empty.txt

llvm/utils/lit/tests/Inputs/diff-test-update/diff-tmp-dir.test

@@ -1,5 +1,7 @@
 # RUN: mkdir %t
-# RUN: touch %S/empty.txt
+# Tests that if file A is in the %t directory and file B is not,
+# the diff test updater copies from file A to B.
+# RUN: echo "" > %S/empty.txt
 # RUN: cp %S/1.in %t/1.txt
 
 # RUN: diff %t/1.txt %S/empty.txt

llvm/utils/lit/tests/diff-test-update.py

@@ -1,10 +1,10 @@
 # RUN: not %{lit} --update-tests -v %S/Inputs/diff-test-update | FileCheck %s
 
-# CHECK: # update-diff-test: could not deduce source and target from {{.*}}/Inputs/diff-test-update/1.in and {{.*}}/Inputs/diff-test-update/2.in
-# CHECK: # update-diff-test: could not deduce source and target from {{.*}}/diff-test-update/Output/diff-bail2.test.tmp/1.txt and {{.*}}/diff-test-update/Output/diff-bail2.test.tmp/2.txt
-# CHECK: # update-diff-test: copied {{.*}}/Output/diff-expected.test.tmp/my-file.txt to {{.*}}/Output/diff-expected.test.tmp/my-file.expected
-# CHECK: # update-diff-test: copied {{.*}}/Output/diff-tmp-dir.test.tmp/1.txt to {{.*}}/Inputs/diff-test-update/empty.txt
-# CHECK: # update-diff-test: copied {{.*}}/Inputs/diff-test-update/Output/diff-tmp.test.tmp.txt to {{.*}}/Inputs/diff-test-update/diff-t-out.txt
+# CHECK: # update-diff-test: could not deduce source and target from {{.*}}1.in and {{.*}}2.in
+# CHECK: # update-diff-test: could not deduce source and target from {{.*}}1.txt and {{.*}}2.txt
+# CHECK: # update-diff-test: copied {{.*}}my-file.txt to {{.*}}my-file.expected
+# CHECK: # update-diff-test: copied {{.*}}1.txt to {{.*}}empty.txt
+# CHECK: # update-diff-test: copied {{.*}}diff-tmp.test.tmp.txt to {{.*}}diff-t-out.txt
 
 
 # CHECK: Failed: 5 (100.00%)