When trying to compile the protobuf code as a DLL, and then compile other DLLs
with generated .pb.cc/h files that reference
InternalMetadataWithArena::InternalMetadataWithArena(Arena*), MSVC gives an
"unresolved external symbol" error.  This seems to be due to the function being
simultaneously exported and inline.  Moving it out-of-line fixes things.

There are other functions exported and inline as well but de-inlining them
doesn't seem to be necessary to get the build working, and I'd rather de-inline
as few functions as possible.

Change-Id: I17e16fdae6e4d2fb74d178fa5564a609ed58af1d

Change-Id: Ieb7a2c2fbf35bc2a8fa65b915a5ecb68c83863e4

cmake targets that depend on these libraries will automatically have
these include directories.

[ci skip]

Change-Id: I9391c09640e0b0b2b21c45a97a1fc91814d95c5d