Purge input relations if pruneImdtRels is set

[adamjseitz]

This PR undoes part of a previous PR that I implemented, #2286. With this PR, when pruneImdtRels is set, .input relations will be purged when they are no longer needed.

In the previous PR, I changed behavior to preserve both input and output relations when the pruneImdtRels option was enabled. However, I think that it was the wrong decision to preserve input relations.

By preserving only output relations, this maximizes the opportunity for memory savings, allowing any relation no longer needed by souffle to be purged. If a C++ program needs to access an input relation again after running the souffle program via the C++ interface, I now realize that the program should have apply both the.input and .output directives, rather than relying on an .input to be preserved. This is more consistent with other Souffle behavior; if an input relation is never used by a Souffle program, Souffle is more than happy to optimize that relation out of the program entirely.

[codecov]

Codecov Report

Merging #2394 (fa18123) into master (0334b07) will increase coverage by 0.10%.
The diff coverage is 85.57%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2394      +/-   ##
==========================================
+ Coverage   77.56%   77.66%   +0.10%     
==========================================
  Files         467      472       +5     
  Lines       30785    31041     +256     
==========================================
+ Hits        23877    24107     +230     
- Misses       6908     6934      +26     

Impacted Files	Coverage Δ
src/parser/ParserDriver.h	100.00% <ø> (ø)
src/synthesiser/Synthesiser.h	100.00% <ø> (ø)
.../ast/transform/SimplifyConstantBinaryConstraints.h	50.00% <50.00%> (ø)
src/MainDriver.cpp	70.39% <70.27%> (+0.15%)	⬆️
src/parser/scanner.ll	78.01% <76.92%> (+5.49%)	⬆️
tests/libsouffle_interface/vfs_overlay_test.cpp	85.93% <85.93%> (ø)
src/parser/SrcLocation.cpp	91.86% <87.50%> (+7.24%)	⬆️
src/parser/ParserDriver.cpp	89.44% <88.33%> (-0.39%)	⬇️
src/parser/VirtualFileSystem.cpp	88.37% <88.37%> (ø)
...st/transform/SimplifyConstantBinaryConstraints.cpp	100.00% <100.00%> (ø)
... and 12 more