Globally force -Wdeprecated-declarations to be a warning.

Some code compiles with -Werror, which makes it very hard to mark
anything as deprecated without breaking the build. This option
will ensure that we can still mark things as deprecated without
breaking such code. Among other things, this will allow us to
undo a local change to protobuf which disables deprecation
warnings.

Bug: 203713560
Test: presubmit
Change-Id: I718de31c3ff685dba4104f3c9e7bd5d169d89f7e