[lldb][ClangExpressionParser] Don't leak memory when multiplexing ExternalASTSources (llvm#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.

(cherry picked from commit 770cd24)

Author: Michael137

lldb/source/Plugins/ExpressionParser/Clang/ASTUtils.h

@@ -30,7 +30,7 @@ namespace lldb_private {
 /// Wraps an ExternalASTSource into an ExternalSemaSource.
 ///
 /// Assumes shared ownership of the underlying source.
-class ExternalASTSourceWrapper : public ImporterBackedASTSource {
+class ExternalASTSourceWrapper : public clang::ExternalSemaSource {
   llvm::IntrusiveRefCntPtr<ExternalASTSource> m_Source;
 
 public: