Cherry-pick bug fixes for php

That implementation of json is not being used - this extension is using
a json encoder/decoder provided by 'upb'.

Use bool deterministic to suppress warning

Fixes #3059 by re-applying 08b1c718 from #3087.

Update version number to 3.3.1

Cherry-pick a bug fix: workaround gcc < 4.5.0 bug

Fix offset type macro to match the tables it is used in

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=189Signed-off-by: Mario J. Rugiero <mrugiero@gmail.com>

<: is a digraph, so a space is necessary for compilation on certain compilers.

Fix several C++ build issues.

Cherry-pick the fix from pull request #2873

Fix compilation on alpine 3.5

Fix python3 issue.

descriptor.proto uses proto2 syntax, which is not ready for external
usage. However, some proto3 files import descriptor.proto and cannot be
used. In this PR, all references (We cheated by only removing
extensions, which is enough for now. User should avoid using messages
defined in descriptor.proto as field type.) to content in
descriptor.proto are removed from generated files. Those that import
descriptor.proto can be used like other proto files.

Fix the bug by sharing the code for generating class name for both
message and enum.