Fix error introduced by changing function signatures.

Since ClangASTSource::layoutRecordType() was overriding a virtual
function in the base, this was inadvertently causing a new method
to be introduced rather than an override.  To fix this all method
signatures are changed back to taking DenseMaps, and the `override`
keyword is added to make sure this type of error doesn't happen
again.

To keep the original fix intact, which is that fields and bases
must be added in offset order, the ImportOffsetMap() function
now copies the DenseMap into a vector and then sorts the vector
on the value type (e.g. the offset) before iterating over the
sorted vector and inserting the items.

llvm-svn: 233099