[utils] add update-verify-tests.py from clang

This script is used to automatically update test cases using -verify in
clang's test suite. Swift also has a similar -verify test functionality,
and while it can automatically fix some of them using -verify-apply-all,
this functionality only updates existing checks - it doesn't add or
remove any, and it doesn't handle newer and more complex things like
expected-expansion. Handling that type of complexity feels out of scope
to embed in the compiler, so let's copy clang's approach.

This commit adds this script as is from clang. It doesn't work at all
for Swift in its current form, as the output from Swift's -verify is
formatted differently than in clang. This will be fixed in subsequent
commits.

This could have been done by adapting the script as-is in the
llvm-project repository (since it only exists in the Swift fork, not
upstream), but tests using swift-frontend would have to reside in the
swift repo, and modifying a script in one repo with tests in a different
repo sounds like a recipe for endless CI issues.

Author: hnrklssn

utils/update-verify-tests.py

@@ -0,0 +1,39 @@
+import sys
+import argparse
+from UpdateVerifyTests.core import check_expectations
+
+"""
+ Pipe output from clang's -verify into this script to have the test case updated to expect the actual diagnostic output.
+ When inserting new expected-* checks it will place them on the line before the location of the diagnostic, with an @+1,
+ or @+N for some N if there are multiple diagnostics emitted on the same line. If the current checks are using @-N for
+ this line, the new check will follow that convention also.
+ Existing checks will be left untouched as much as possible, including their location and whitespace content, to minimize
+ diffs. If inaccurate their count will be updated, or the check removed entirely.
+
+ Missing features:
+  - multiple prefixes on the same line (-verify=my-prefix,my-other-prefix)
+  - multiple prefixes on separate RUN lines (RUN: -verify=my-prefix\nRUN: -verify my-other-prefix)
+  - regexes with expected-*-re: existing ones will be left untouched if accurate, but the script will abort if there are any
+    diagnostic mismatches on the same line.
+  - multiple checks targeting the same line are supported, but a line may only contain one check
+  - if multiple checks targeting the same line are failing the script is not guaranteed to produce a minimal diff
+
+Example usage:
+  clang -verify [file] | python3 update-verify-tests.py
+  clang -verify=check [file] | python3 update-verify-tests.py --prefix check
+"""
+
+
+def main():
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument(
+        "--prefix", default="expected", help="The prefix passed to -verify"
+    )
+    args = parser.parse_args()
+    output = check_expectations(sys.stdin.readlines(), args.prefix)
+    print(output)
+
+
+if __name__ == "__main__":
+    main()
+