[clang] [clang-repl] Set up executor implicitly to account for init PTUs (PR #84758)

[Stefan Gränitz via cfe-commits]

weliveindetail wrote:

@vvereschaka Thanks for the note! should be fixed now.

> I am not sure how this patch changed these tests to start failing on Windows. Do you have any clue?

It's just cases I missed in the first quick-fix

> What do you mean?

This is adding a lot of boilerplate, because each and ever test now looks like this:
```
#ifdef CLANG_INTERPRETER_PLATFORM_CANNOT_CREATE_LLJIT
TEST(CodeCompletionTest, DISABLED_TemplateFunctions) {
#else
 TEST(CodeCompletionTest, TemplateFunctions) {
#endif
  if (!HostSupportsJit())
    GTEST_SKIP();
  <<< actual test >>>
}
```

@vgvassilev LIT tests check host JIT support during config and bulk skip all affected tests. (1) Maybe we can do sth like this in unittests as well? Or otherwise: (2) Add an option to the Interpreter to skip init PTUs and roll-back my quick fixes? What do you think?

https://github.com/llvm/llvm-project/pull/84758