FormatID_autogen.h's enums' order was altered by CL
https://chromium-review.googlesource.com/c/angle/angle/+/2413165
However, Metal default shaders relied on old order thus the result
PBO related computation based on enum value has become wrong.

This CL reuse gen_angle_format_table.gen_enum_string() function to
generate format enums for both
- src/libANGLE/renderer/FormatID_autogen.h (used by C++)
- src/libANGLE/renderer/metal/shaders/format_autogen.h (used by Metal
  default shaders)

Bug: angleproject:2634
Change-Id: I52fad168d1932e0386dcf789574f06d341dbd9c3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2418500
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>