[clang] [clang][dataflow] Add `Environment::initializeFieldsWithValues()`. (PR #81239)

[via cfe-commits]

martinboehme wrote:

Reverting: Causes build errors.

```
/var/lib/buildbot/sanitizer-buildbot6/sanitizer-x86_64-linux-android/build/llvm-project/clang/lib/Analysis/FlowSensitive/DataflowEnvironment.cpp:942:19: error: loop variable '[Field, FieldLoc]' creates a copy from type 'const llvm::detail::DenseMapPair<const clang::ValueDecl *, clang::dataflow::StorageLocation *>' [-Werror,-Wrange-loop-construct]
  942 |   for (const auto [Field, FieldLoc] : Loc.children()) {
```

https://github.com/llvm/llvm-project/pull/81239