fix mutually dependent aggregates semantics checks

[quentin]

Hi,
I think this change fixes the semantics checks on mutually dependent aggregates.

The checker was looking for mutually dependent aggregates between pairs of aggregate only comparing (un)bound variables by name, even when they do not have the same parent clause.

For instance in this program, where rules (R1, R2) are independent from (R3, R4):

/// An aggregate with R1 & R2

.decl R1(i:number, j:number, h:number)
.input R1()

.decl R2(i:number, j:number, v:number)
R2(i, j, v) :-
  R1(i, j, _),
  v = count : { R1(x, j, h), i < x, h > 2 }.

.output R2()

/// An aggregate with R3 & R4

.decl R3(i:number, j:number, h:number)
.input R3()

.decl R4(i:number, j:number, v:number)
R4(i, j, v) :-
  R3(i, j, _),
  v = count : { R3(i, y, h), j < y, h > 2 }.

.output R4()

Would error out:

Error: Mutually dependent aggregate in file foo.dl at line 9
  v = count : { R1(x, j, h), i < x, h > 2 }.
------^--------------------------------------
Error: Mutually dependent aggregate in file foo.dl at line 21
  v = count : { R3(i, y, h), j < y, h > 2 }.
------^--------------------------------------

[codecov]

Codecov Report

Merging #2372 (d6a7a54) into master (b311946) will decrease coverage by 0.01%.
The diff coverage is 92.45%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2372      +/-   ##
==========================================
- Coverage   77.52%   77.50%   -0.02%     
==========================================
  Files         466      467       +1     
  Lines       30690    30759      +69     
==========================================
+ Hits        23791    23840      +49     
- Misses       6899     6919      +20     

Impacted Files	Coverage Δ
src/synthesiser/GenDb.h	93.75% <78.26%> (-6.25%)	⬇️
src/MainDriver.cpp	70.23% <92.81%> (ø)
src/ast/transform/SemanticChecker.cpp	97.04% <100.00%> (-0.01%)	⬇️
src/souffle.cpp	100.00% <100.00%> (ø)
src/synthesiser/GenDb.cpp	97.15% <100.00%> (+0.05%)	⬆️
...ouffle/datastructure/ConcurrentInsertOnlyHashMap.h	87.40% <0.00%> (-1.58%)	⬇️
src/include/souffle/utility/ParallelUtil.h	83.72% <0.00%> (-1.56%)	⬇️
src/reports/DebugReport.cpp	20.19% <0.00%> (-0.97%)	⬇️
src/ram/analysis/Index.cpp	89.71% <0.00%> (-0.59%)	⬇️
src/include/souffle/datastructure/BTree.h	97.91% <0.00%> (-0.42%)	⬇️
... and 3 more

[XiaowenHu96]

LGTM