Update guides to use brstack, with a mention to BRBE for AArch64. Use
brstack in user-facing outputs.

---------

Co-authored-by: Amir Ayupov <aaupov@fb.com>

https://github.com/codespell-project/codespell
```bash
codespell bolt --skip="*.yaml,Maintainers.txt" --write-changes \
    --ignore-words-list=acount,alledges,ans,archtype,defin,iself,mis,mmaped,othere,outweight,vas
```

Record the number of function invocations from external code - code
outside the binary, which may include JIT code and DSOs. Accounting
external entry counts improves the fidelity of call graph flow 
conservation analysis.

Test Plan: updated shrinkwrapping.test

We can use try_emplace to succinctly implement GetOrCreateFuncEntry
and GetOrCreateFuncMemEntry.  Since it's a bit mouthful to say
FuncBasicSampleData::ContainerTy(), this patch changes the second
parameters to default ones.

Sample is a general term covering both basic (IP) and branch (LBR)
profiles. Find and replace ambiguous uses of sample in a basic sample
sense.

Rename `RawBranchCount` into `RawSampleCount` reflecting its use for
both kinds of profile.

Rename `PF_LBR` profile type as `PF_BRANCH` reflecting non-LBR based
branch profiles (non-brstack SPE, synthesized brstack ETM/PT).

Follow-up to #137644.

Test Plan: NFC

For profile with LBR samples, binary function profile density is
computed as a ratio of executed bytes to function size in bytes.

For profile with IP samples, use the size of basic block containing the
sample IP as a numerator.

Test Plan: updated perf_test.test

I'm planning to remove StringRef::equals in favor of
StringRef::operator==.

- StringRef::operator==/!= outnumber StringRef::equals by a factor of
  276 under llvm-project/ in terms of their usage.

- The elimination of StringRef::equals brings StringRef closer to
  std::string_view, which has operator== but not equals.

- S == "foo" is more readable than S.equals("foo"), especially for
  !Long.Expression.equals("str") vs Long.Expression != "str".