[BasicBlockSections] Introduce the path cloning profile format to...

[BasicBlockSections] Introduce the path cloning profile format to BasicBlockSectionsProfileReader. (#67214)

Following up on prior RFC
(https://lists.llvm.org/pipermail/llvm-dev/2020-September/145357.html)
we can now improve above our highly-optimized basic-block-sections
binary (e.g., 2% for clang) by applying path cloning. Cloning can
improve performance by reducing taken branches.

This patch prepares the profile format for applying cloning actions.

The basic block cloning profile format extends the basic block sections
profile in two ways.

1. Specifies the cloning paths with a 'p' specifier. For example, `p 1 4
5` specifies that blocks with BB ids 4 and 5 must be cloned along the
edge 1 --> 4.
2. For each cloned block, it will appear in the cluster info as
`<bb_id>.<clone_id>` where `clone_id` is the id associated with this
clone.

For example, the following profile specifies one cloned block (2) and
determines its cluster position as well.
```
f foo
p 1 2
c 0 1 2.1 3 2 5
```

This patch keeps backward-compatibility (retains the behavior for old
profile formats). This feature is only introduced for profile version >=
1.