[clang][ASTImporter] Add import API for 'const Type *' (NFC).

There was only an `Import` function for `QualType` but not for `Type`.
For correct import of some AST nodes where not `QualType` is used
an import of `Type *` is needed. (It is the case with
`FieldDecl::getCapturedVLAType`.)

Reviewed By: shafik, teemperor, martong

Differential Revision: https://reviews.llvm.org/D98951