[clang][dataflow] Remove `declToLocConsistent()` assertion. (#69819)

As described [here](https://discourse.llvm.org/t/70086/6), there are
legitimate
non-bug scenarios where two `DeclToLoc` maps to be joined contain
different
storage locations for the same declaration. This patch also adds a test
containing an example of such a situation. (The test fails without the
other
changes in this patch.)

With the assertion removed, the existing logic in `intersectDenseMaps()`
will
remove the corresponding declaration from the joined DeclToLoc map.

We also remove `removeDecl()`'s precondition (that the declaration must
be
associated with a storage location) because this may no longer hold if
the
declaration was previously removed during a join, as described above.