F2dace/fix local consts data ref

[Mittagskogel]

This PR extends the capabilities of the _track_local_consts() AST desugaring transformation.

By introducing a new spec generator find_indexed_dataref_component_spec(), we enable constant replacement of Part_Refs and Data_Refs like a(5) or a % b(4) % c(7,3). These Data_Refs get turned into specs like ('main', ('a', 5)) or (('main', 'a'), (a, ('b', 4), ('c', 7, 3)). Furthermore, the constant tracker can detect and replace array pointer references:

SUBROUTINE foo(var)
REAL, TARGET arr(4, 6)
REAL, POINTER arrptr
REAL, INTENT(IN) :: var

arrptr => arr(:,:)
arrptr(2,3) = var  ! This gets replaced with arr(2,3) = var
END SUBROUTINE foo

Additionally, we fix a bug in which variable pointer assignments are propagated. For example:

SUBROUTINE bar(idx)
REAL, TARGET :: arr(6)
REAL, POINTER :: ptr

REAL :: var
INTEGER, INTENT(IN) :: idx

ptr => arr(idx)
idx = idx + 1
var = ptr  ! arr(idx) cannot be propagated here because idx has changed!
END SUBROUTINE bar

The fix checks whether Data_Ref nodes on the RHS of the pointer assignment contain any variables.