[flang] Implemented 2 Semantic checks for DATA statement and fixed a few bugs

Summary
  - Implemented C876, C877
  - Fixed IsConstantExpr to check C879
  - Fixed bugs in few test cases - data01.f90, block-data01.f90,
  pre-fir-tree02.f90
  - Modified implementation of C8106 to identify all automatic objects
  and modified equivalence01.f90 to reflect the changes

Differential Revision: https://reviews.llvm.org/D78424