Add unspport.

This is relate #101101

This is only for struct containing nested structs with user defined
mappers.

Add four functions:
1>buildImplicitMap: build map for default mapper
2>buildImplicitMapper:  build default mapper.
3>hasUserDefinedMapper for given mapper name and mapper type, lookup
user defined map, if found one return true.
4>isImplicitMapperNeeded check if Mapper is needed

During create map, in checkMappableExpressionList, call
isImplicitMapperNeeded when it return true, call buildImplicitMapper to
generate implicit mapper and added to map clause.

https://github.com/llvm/llvm-project/pull/101101