[lldb] Add asserts that prevent construction of cycles in the decl origin tracking

LLDB tracks where any imported `clang::Decl` originally came from via a simple
map from 'imported decl' to 'original decl'. That information is used to later
complete parts of the Decl when more information is requested about a certain
Decl (e.g., via the ExternalASTSource interface from Clang).

When finding the 'original decl' for a given decl, the ASTImporterDelegate
essentially just recursively follows the previously mentioned map from
'imported' to 'original decl' until it can find any further 'original decl'. The
final found decl is then the one that will be imported. The recursion is
necessary as in LLDB we don't just import decls from one ASTContext to another,
but also from one ASTContext to another via a (potentially temporary)
ASTContext. For example, the expression parser creates a temporary ASTContext
for parsing the current expression.

The problem with the recursion is however that if we somehow get a cycle into
our mapping, then the ASTImporterDelegate will just infinite recurse. As the
infinite recursion usually happens after the cycle was already created in a code
path such as completing a type, the crash backtraces we get for these bugs are
not very useful. However having the backtrace where the faulty map entry is
created usually makes the code trivial to fix (as there should be some rogue
CopyType call or something similar nearby. See for example D96366).

This patch tries to make these issues easier to track down by putting a bunch of
sanity asserts in the code that fills out the map. All the asserts are just
checking that there is no direct cycle (ASTContext maps to itself) when updating
the origin tracking map.

The assert in the ASTImportDelegate constructor is an `lldbassert` (which also
is getting checked in release builds with disabled asserts) as the code path
there is pretty cold and we can reliably detect a rogue CopyType call from
there.

I also had to update some code in
`ClangASTImporter::ASTImporterDelegate::Imported`. This code already had a
safety check for creating a cycle in the origin tracking map, but it still
constructed an ASTImporter while checking for the cycle (by requesting a
delegate via `GetDelegate` and passing two identical ASTContexts which looks
like a rogue CopyType call to the checks).

Reviewed By: shafik

Differential Revision: https://reviews.llvm.org/D97300