[compiler-rt] [llvm] [ctxprof] Capture sampling info for context roots (PR #131201)

[Erich Keane via llvm-commits]

erichkeane wrote:

Using clang-10 as a host compiler (and GCC/stdlibc++ 9.4, both are part of our support matrix: https://llvm.org/docs/GettingStarted.html#host-c-toolchain-both-compiler-and-standard-library), I'm getting the following error that I think is from this patch (at least based on the warning): 

```
In file included from /local/home/ekeane/llvm-project/llvm/lib/Transforms/Utils/InlineFunction.cpp:26:
In file included from /local/home/ekeane/llvm-project/llvm/include/llvm/Analysis/CtxProfAnalysis.h:17:
/local/home/ekeane/llvm-project/llvm/include/llvm/ProfileData/PGOCtxProfReader.h:109:3: warning: explicitly defaulted default constructor is implicitly deleted [-Wdefaulted-function-deleted]
  PGOCtxProfContext() = default;
  ^
/local/home/ekeane/llvm-project/llvm/include/llvm/ProfileData/PGOCtxProfReader.h:95:33: note: default constructor of 'PGOCtxProfContext' is implicitly deleted because field 'RootEntryCount' of const-qualified type 'const std::optional<uint64_t>' (aka 'const optional<unsigned long>') would not be initialized
  const std::optional<uint64_t> RootEntryCount;
                                ^
In file included from /local/home/ekeane/llvm-project/llvm/lib/Transforms/Utils/InlineFunction.cpp:26:
/local/home/ekeane/llvm-project/llvm/include/llvm/Analysis/CtxProfAnalysis.h:33:5: error: call to implicitly-deleted default constructor of 'llvm::PGOCtxProfContext'
    FunctionInfo(StringRef Name) : Name(Name) {}
    ^
/local/home/ekeane/llvm-project/llvm/include/llvm/ProfileData/PGOCtxProfReader.h:109:3: note: explicitly defaulted function was implicitly deleted here
  PGOCtxProfContext() = default;
  ^
/local/home/ekeane/llvm-project/llvm/include/llvm/ProfileData/PGOCtxProfReader.h:95:33: note: default constructor of 'PGOCtxProfContext' is implicitly deleted because field 'RootEntryCount' of const-qualified type 'const std::optional<uint64_t>' (aka 'const optional<unsigned long>') would not be initialized
  const std::optional<uint64_t> RootEntryCount;
```

Min-repro on godbolt is here: https://godbolt.org/z/65TdreMaa

Looks like this repros up until Clang 11.  

I found I can fix this by just doing an explicit default init with curleys on the type.  I've done a RAC patch here: <2e6402ca2c6c33ccf41d74383a8e3afb82489410>

Feel free to correct this in a different way if this doesn't suffice for you.

https://github.com/llvm/llvm-project/pull/131201