[OptRemark] Move YAML writing to IR

Before the patch this was in Analysis.  Moving it to IR and making it implicit
part of LLVMContext::diagnose allows the full opt-remark facility to be used
outside passes e.g. the pass manager.  Jessica is planning to use this to
report function size after each pass.  The same could be used for time
reports.

Tested with BUILD_SHARED_LIBS=On.

llvm-svn: 314909