[Remarks] Add two serialization modes for remarks: separate and standalone

[Remarks] Add two serialization modes for remarks: separate and standalone

The default mode is separate, where the metadata is serialized
separately from the remarks.

Another mode is the standalone mode, where the metadata is serialized
before the remarks, on the same stream.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@367328 91177308-0d34-0410-b5e6-96231b3b80d8