[RemarkUtil] Refactor remarkutil tool to use a command registry.

In preperation to move all remark utilities into one tool. We use
command registry to breakdown each utility into a separate file.

For now we have 3 utilities for remarks
1. Convert: which is responsible for converting yaml remarks to
   bitstream and vice-versa
2. Count: Analyse remarks and report count. This currently only supports
   asm-remarks and annotation-summary remarks.
3. Diff remarks: Currently we only have a diff for size remarks using
   `llvm-remark-size-diff`

The first two utilites have been simplified and seperated into two
files. The following commit will move `llvm-remark-size-diff` and fold
it to be inside `llvm-remarkutil` as a subcommand

Differential Revision: https://reviews.llvm.org/D156416

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