[Clnag] Prefer octal to hex for printf

The printf implementation on MacOS that gets picked up by the internal
shell does not support the \xff form as it is not mandated by POSIX.
Prefer octal which is supported and mandated by POSIX.

This is similar to c745c54.

Author: boomanaiden154

clang/test/Lexer/minimize_source_to_dependency_directives_utf8bom.c

@@ -1,5 +1,5 @@
 // Test UTF8 BOM at start of file
-// RUN: printf '\xef\xbb\xbf' > %t.c
+// RUN: printf '\357\273\277' > %t.c
 // RUN: echo '#ifdef TEST\n' >> %t.c
 // RUN: echo '#include <string>' >> %t.c
 // RUN: echo '#endif' >> %t.c