[clang-format] Support inheriting from more than 1 parents in the fallback case

Currently, we are unable to inherit from a chain of parent configs where the outermost parent config has `BasedOnStyle: InheritParentConfig` set. This patch adds a test case for this scenario, and adds support for it.

To illustrate, suppose we have the following directory structure:
```
- e/
  |- .clang-format (BasedOnStyle: InheritParentConfig) <-- outermost config
  |- sub/
    |- .clang-format (BasedOnStyle: InheritParentConfig)
    |- sub/
      |- .clang-format (BasedOnStyle: InheritParentConfig)
      |- code.cpp
```
Now consider what happens when we run `clang-format --style=file /e/sub/sub/code.cpp`.

Without this patch, on a release build, only the innermost config will be applied. On a debug build, clang-format crashes due to an assertion failure.
With this patch, clang-format behaves as we'd expect, applying all 3 configs.

Reviewed By: HazardyKnusperkeks, curdeius

Differential Revision: https://reviews.llvm.org/D116371