[dev-script] Add a new script called at-to-filelist.

This script in combination with <(...) can be used to speed up creating
reproductions from command lines that use '@' files to define their inputs.

An example use case:

Consider a swiftc command line that uses the @ symbol.

  swiftc @/foo/bar/baz.txt

When this is run, the @ command is expanded into a filelist in a temporary
file. This doesn't work with -### since -### outputs the command line with a
temporary file for the filelist, but uses a path to a temporary file that
doesn't exist, e.g.:

  swift -frontend -filelist /tmp/tmp.filelist ...

To run this command, you use the at-to-filelist command as follows:

  swift -frontend -filelist <(at-to-filelist /foo/bar/baz.txt)

Author: gottesmm

utils/dev-scripts/at-to-filelist

@@ -0,0 +1,35 @@
+#!/bin/bash
+
+# Transform an @ file into a file list.
+#
+# The intended use case is given an @ path passed to swift, use this script and
+# bash to automagically expand the @ path by transforming:
+#
+# @/foo/bar/baz.txt
+#
+#   =>
+#
+# <(at-to-filelist /foo/bar/baz.txt)
+#
+# For those unaware, <(...) causes bash to output the subshell's output into a
+# file and then replace <(...) with the path to that temporary file.
+#
+# Example:
+#
+# Consider a swiftc command line that uses the @ symbol.
+#
+#   swiftc @/foo/bar/baz.txt
+#
+# When this is run, the @ command is expanded into a filelist in a temporary
+# file. This doesn't work with -### since -### outputs the command line with a
+# temporary file for the filelist, but uses a path to a temporary file that
+# doesn't exist, e.g.:
+#
+#   swift -frontend -filelist /tmp/tmp.filelist ...
+#
+# To run this command, you use the at-to-filelist command as follows:
+#
+#   swift -frontend -filelist <(at-to-filelist /foo/bar/baz.txt)
+cat ${1} | tr -s ";" "\n"
+
+echo