`CounterPair` can hold `<uint32_t, uint32_t>` instead of current
`unsigned`, to hold also the counter number of SkipPath. For now, this
change provides the skeleton and only `CounterPair::Executed` is used.

Each counter number can have `None` to suppress emitting counter
increment. 2nd element `Skipped` is initialized as `None` by default,
since most `Stmt*` don't have a pair of counters.

This change also provides stubs for the verifier. I'll provide the impl
of verifier for `+Asserts` later.

`markStmtAsUsed(bool, Stmt*)` may be used to inform that other side
counter may not emitted.

`markStmtMaybeUsed(S)` may be used for the `Stmt` and its inner will be
excluded for emission in the case of skipping by constant folding. I put
it into places where I found.

`verifyCounterMap()` will check the coverage map and the counter map,
and can be used to report inconsistency.

These verifier methods shall be eliminated in `-Asserts`.


https://discourse.llvm.org/t/rfc-integrating-singlebytecoverage-with-branch-coverage/82492

Identified with misc-include-cleaner.

Don't call raw_string_ostream::flush(), which is essentially a no-op.
As specified in the docs, raw_string_ostream is always unbuffered

This pull request is a revised version of #76587. This pull request
fixes some build issues that were present in the previous version of
this change.

> This pull request is the first part of an ongoing effort to extends
PGO instrumentation to GPU device code. This PR makes the following
changes:
>
> - Adds blank registration functions to device RTL
> - Gives PGO globals protected visibility when targeting a supported
GPU
> - Handles any addrspace casts for PGO calls
> - Implements PGO global extraction in GPU plugins (currently only
dumps info)
>
> These changes can be tested by supplying `-fprofile-instrument=clang`
while targeting a GPU.

With `system-headers-coverage=false`, functions defined in system
headers were not instrumented but corresponding covmaps were emitted. It
caused wasting covmap and profraw.

This change improves:

- Reduce object size (due to reduced covmap)
- Reduce size of profraw (uninstrumented system headers occupied
counters)
- Smarter view of coverage report. Stubs of uninstrumented system
headers will be no longer seen.

Part of #97952

This reverts commit 5fd2af38e461445c583d7ffc2fe23858966eee76. It caused build issues and broke the buildbot.

This pull request is the first part of an ongoing effort to extends PGO
instrumentation to GPU device code. This PR makes the following changes:

- Adds blank registration functions to device RTL
- Gives PGO globals protected visibility when targeting a supported GPU
- Handles any addrspace casts for PGO calls
- Implements PGO global extraction in GPU plugins (currently only dumps
info)

These changes can be tested by supplying `-fprofile-instrument=clang`
while targeting a GPU.

A C array can be implicitly cast to ArrayRef.