[test] Use dd instead of head -c.

head -c, to print the specified number of bytes, is a GNU extension.
Using dd for the same task should be somewhat more portable.

Mac OS X does not have status=none, so redirect the status output away
instead. Despite appearances, redirecting with the Bourne shell-ism
`2>/dev/null` is fine on Windows since Windows executes with the
internal lit shell runner, which translates `/dev/null` to a temporary
file.

Author: 3405691582

test/DebugInfo/modulecache.swift

@@ -9,7 +9,7 @@ import ClangModule
 
 // RUN: %empty-directory(%t)
 // RUN: %swift-ide-test_plain -print-usrs -target %target-triple -module-cache-path %t  -I %S/Inputs -source-filename %s
-// RUN: head -c 4 %t/*/ClangModule-*.pcm | grep -q CPCH
+// RUN: dd bs=1 count=4 < %t/*/ClangModule-*.pcm 2>/dev/null | grep -q CPCH
 
 // 2. Test that swift is creating clang modules with debug info.