[llvm][mustache] Use BumpPtrAllocator to save ASTNodes (#159194)

We make the Mustache ASTNodes usable in the arena by first removing all
of the memory owning data structures, like std::vector, std::unique_ptr,
and SmallVector. We use standard LLVM list types to hold this data
instead, and make use of a UniqueStringSaver to hold the various
templates strings.

Additionally, update clang-doc APIs to use the new interfaces.

Future work can make better use of Twine interfaces to help avoid any
intermediate copies or allocations.

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