[clang] [clang-repl] Rework layering of incremental executors. (PR #175448)

[via cfe-commits]

mcbarton wrote:

@vgvassilev When trying to do an Emscripten build of llvm against the branch of this PR I get the following error

```
/Users/username/llvm-pr-test/llvm-project/clang/lib/Interpreter/Wasm.cpp:62:7: error: no matching constructor for initialization of 'IncrementalExecutor'
   62 |     : IncrementalExecutor(TSC) {}
      |       ^                   ~~~
/Users/username/llvm-pr-test/llvm-project/clang/include/clang/Interpreter/IncrementalExecutor.h:75:7: note: candidate constructor (the implicit copy constructor) not viable: cannot convert argument of incomplete type 'llvm::orc::ThreadSafeContext' to 'const IncrementalExecutor' for 1st argument
   75 | class IncrementalExecutor {
      |       ^~~~~~~~~~~~~~~~~~~
/Users/username/llvm-pr-test/llvm-project/clang/include/clang/Interpreter/IncrementalExecutor.h:75:7: note: candidate constructor (the implicit default constructor) not viable: requires 0 arguments, but 1 was provided
/Users/username/llvm-pr-test/llvm-project/clang/lib/Interpreter/Wasm.cpp:163:21: error: expected '}'
  163 | } // namespace clang
      |                     ^
/Users/username/llvm-pr-test/llvm-project/clang/lib/Interpreter/Wasm.cpp:58:17: note: to match this '{'
   58 | namespace clang {
      |                 ^

```

https://github.com/llvm/llvm-project/pull/175448