[Remarks] Emit a section containing remark diagnostics metadata

[Remarks] Emit a section containing remark diagnostics metadata

A section containing metadata on remark diagnostics will be emitted if
the flag (-mllvm) -remarks-section is present.

For now, the metadata is:

* a magic number for remarks: "REMARKS\0"
* the version number: a little-endian uint64_t
* the absolute file path to the serialized remark diagnostics: a
  null-terminated string.

Differential Revision: https://reviews.llvm.org/D59571

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@357043 91177308-0d34-0410-b5e6-96231b3b80d8