[flang] Semantic checks for C702

[flang] Semantic checks for C702
C702 (R701) A colon shall not be used as a type-param-value except in the
declaration of an entity that has the POINTER or ALLOCATABLE attribute.

I added code to the visitor for a TypeDeclarationStmt to check for the
'LEN' type parameter for strings and to loop over the type parameters
for derived types.

I also ran into a few situations where previous tests had erroneously
used a colon for type parameters without either the POINTER or
ALLOCATABLE attribute and fixed them up.

Original-commit: flang-compiler/f18@a1a95bfcd1811d697f1386fe57af664b8a16ffb5
Reviewed-on: https://github.com/flang-compiler/f18/pull/973