[lldb][ClangExpressionParser] Don't leak memory when multiplexing ExternalASTSources (#104799) When we use `SemaSourceWithPriorities` as the `ASTContext`s ExternalASTSource, we allocate a `ClangASTSourceProxy` (via `CreateProxy`) and two `ExternalASTSourceWrapper`. Then we push these sources into a vector in `SemaSourceWithPriorities`. The allocated `SemaSourceWithPriorities` itself will get properly deallocated because the `ASTContext` wraps it in an `IntrusiveRefCntPtr`. But the three sources we allocated earlier will never get released. This patch fixes this by mimicking what `MultiplexExternalSemaSource` does (which is what `SemaSourceWithPriorities` is based on anyway). I.e., when `SemaSourceWithPriorities` gets constructed, it increments the use count of its sources. And on destruction it decrements them. Similarly, to make sure we dealloacted the `ClangASTProxy` properly, the `ExternalASTSourceWrapper` now assumes shared ownership of the underlying source.
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