* Added -Wextra and -Wshorten-64-to-32 and fixed resulting errors.
* Disable -Wshorten-32-to-64 since Kokoro is missing Clang.
* Fixed -Wextra warnings for gcc.
* Reordered UPB_UNUSED() to come after declarations.
* Added another -pedantic fix and log CC version.
* Fix compile error and conditionally run use_bazel.sh.
* Moved set -e after use_bazel.sh.
* Fixed typo in conditional.
- A new PHP-specific upb amalgamation. It contains everything related to upb_msg, but leaves out all of the old handlers-related interfaces and encoders/decoders.
# Schema/Defs Changes
- Changed `upb_fielddef_msgsubdef()` and `upb_fielddef_enumsubdef()` to return `NULL` instead of assert-failing if the field is not a message or enum.
- Added `upb_msgdef_iswrapper()`, to test whether this is a wrapper well-known type.
# Decoder
- Decoder bugfix: when we parse a submessage inside a oneof, we need to clear out any previous data, so we don't misinterpret it as a pointer to an existing submessage.
# JSON Decoder
- Allowed well-known types at the top level to have their special processing.
- Fixed a bug that could occur when parsing nested empty lists/objects, eg `[[]]`.
- Made the "ignore unknown" option also be permissive about unknown enumerators by setting them to 0.
# JSON Encoder
- Allowed well-known types at the top level to have their special processing.
- Removed all spaces after `:` and `,` characters, to match the old encoder and pass goldenfile tests.
# Message / Reflection
- Changed `upb_msg_hasoneof()` -> `upb_msg_whichoneof()`. The new function returns the `upb_fielddef*` of whichever oneof is set.
- Implemented `upb_msg_clearfield()` and added/implemented `upb_msg_clear()`.
- Added `upb_msg_discardunknown()`. Part of me thinks this should go in a util library instead of core reflection since it is a recursive algorithm.
# Compiler
- Always emit descriptors as an array instead of as a string, to avoid exceeding maximum string lengths. If this becomes a speed issue later we can go back to two separate paths.
Joshua Haberman