[clang AST] move mangling API to namespace clang to allow calls from swift-frontend (#137884)

When implementing `@objcDirect` in Swift, Swift needs to mangle a native
`Decl` that is not a clang Node, which in turn don't have access to
`clang::MangleContext`. Reimplementing mangling logic in Swift is
redundant.

This patch moves mangling logic from `clang::MangleContext` to `clang`
using only basic types (`StringRef`, `std::optional`, etc.), such that
Swift can we can just call Clang API: Swift depends on Clang already.

We are separating this from #126639 so we can draft the proposal on the
Swift side. #126639 will be worked to depend on this PR.

Tests: No new tests, old ones should pass with no problem.

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Signed-off-by: Peter Rong <PeterRong@meta.com>

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