[clang][modules] Use extensible RTTI for ModuleFileExtension

Clang exposes an interface for extending the PCM/PCH file format: `ModuleFileExtension`.

Clang itself has only a single implementation of the interface: `TestModuleFileExtension` that can be instantiated via the `-ftest-module-file_extension=` command line argument (and is stored in `FrontendOptions::ModuleFileExtensions`).

Clients of the Clang library can extend the PCM/PCH file format by pushing an instance of their extension class to the `FrontendOptions::ModuleFileExtensions` vector.

When generating the `-ftest-module-file_extension=` command line argument from `FrontendOptions`, a downcast is used to distinguish between the Clang's testing extension and other (client) extensions.

This functionality is enabled by LLVM-style RTTI. However, this style of RTTI is hard to extend, as it requires patching Clang (adding new case to the `ModuleFileExtensionKind` enum).

This patch switches to the LLVM RTTI for open class hierarchies, which allows libClang users (e.g. Swift) to create implementations of `ModuleFileExtension` without patching Clang. (Documentation of the feature: https://llvm.org/docs/HowToSetUpLLVMStyleRTTI.html#rtti-for-open-class-hierarchies)

Reviewed By: artemcm

Differential Revision: https://reviews.llvm.org/D97702

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