[JSONImporter] misses checks whether the data it imports makes sense.

[JSONImporter] misses checks whether the data it imports makes sense.

Without this patch, the JSONImporter did not verify if the data it loads
were correct or not  (Bug llvm.org/PR32543). I add some checks in the
JSONImporter class and some test cases.

Here are the checks (and test cases) I added :

JSONImporter::importContext
- The "context" key does not exist.
- The context was not parsed successfully by ISL.
- The isl_set has the wrong number of parameters.
- The isl_set is not a parameter set.

JSONImporter::importSchedule
- The "statements" key does not exist.
- There is not the right number of statement in the file.
- The "schedule" key does not exist.
- The schedule was not parsed successfully by ISL.

JSONImporter::importAccesses
- The "statements" key does not exist.
- There is not the right number of statement in the file.
- The "accesses" key does not exist.
- There is not the right number of memory accesses in the file.
- The "relation" key does not exist.
- The memory access was not parsed successfully by ISL.

JSONImporter::areArraysEqual
- The "type" key does not exist.
- The "sizes" key does not exist.
- The "name" key does not exist.

JSONImporter::importArrays
/!\ Do not check if there is an key name "arrays" because it is not
considered as an error.
All checks are already in place or implemented in
JSONImporter::areArraysEqual.

Contributed-by: Nicolas Bonfante <nicolas.bonfante@insa-lyon.fr>
Differential Revision: https://reviews.llvm.org/D32739

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@303759 91177308-0d34-0410-b5e6-96231b3b80d8