Add -Wmissing-prototypes and fix issues it finds. (#4019)

Most of these are places where we failed to include a header file and
simply never got an error about this. The fix is to include the header
file.

Most other cases are functions that should have been marked `static` but
were not. Finding all of these was a main motivation for me enabling the
warning despite how much work it is.

One complicating factor was that we weren't including the `handle.h` for
all the state-based handler functions. While this isn't a tiny amount of
code, it is just declarations and doesn't add any extra dependencies. It
also lets us have the checking for which functions need to be `static`
and which don't. For the `parse` library I had to add the `handle.h`
header as well, I tried to match the design of it in `check`.

I have also had to work around a bug in the warning, but given the value
it seems to be providing, that seems reasonable. I've filed the bug
upstream: https://github.com/llvm/llvm-project/issues/94138

I also had to use some hacks to work around limitations of Bazel rules
that wrap `cc_library` rules and don't expose `copts`. I filed a bug for
`cc_proto_library` specifically:
~https://github.com/bazelbuild/bazel/issues/22610~ 
https://github.com/bazelbuild/bazel/issues/4446

toolchain/parse/handle_array_expr.cpp

@@ -5,6 +5,7 @@
 #include "toolchain/lex/token_kind.h"
 #include "toolchain/lex/tokenized_buffer.h"
 #include "toolchain/parse/context.h"
+#include "toolchain/parse/handle.h"
 #include "toolchain/parse/node_kind.h"
 #include "toolchain/parse/state.h"