Make GlobalStoreAndRef consistent in usage inside and outside of loops

[PetroZarytskyi]

The purpose of this PR is to make GlobalStoreAndRef work more consistently inside and outside of loops. The issue is well described in #431. The solution proposed in this PR is as follows:
When GlobalStoreAndRef is called inside a loop, it adds a push statement (clad::push(_t0, x)) to the forward block and a pop statement (clad::pop(_t0)) to the reverse block. The return type of GlobalStoreAndRef is changed from StmtDiff to clang::Expr*. The return value is the top element on the tape (clad::back(_t0)). When GlobalStoreAndRef is called outside a loop, the behavior is essentially the same as before except now it returns a single reference to the created variable ref instead of StmtDiff(ref, ref).
For example (when used to store the condition of an if-stmt inside a loop):

 // forward sweep:
clad::push(_cond1, another_choice > 1);
if (clad::back(_cond1)) {
    ...
}

 // reverse sweep:
if (clad::back(_cond1)) {
    ...
}
clad::pop(_cond1);

With this approach, we automatically ensure safe tape pushes/pops when working with return/break/continue statements, which wasn't the case previously. Because of this a lot of the code used to handle those statements got simplified.
Also, the change of the return type makes GlobalStoreAndRef consistent in usage with StoreAndRef.

Note: I made GlobalStoreAndRef return clad::back(_t0) instead of stored values of clad::push(_t0) and clad::pop(_t0) (as proposed by @parth-07 in #431) because in all use cases _r0 = clad::pop(_t0) was added to the reverse block after _r0 was used (because reverse blocks are reversed). Getting the statement order right would make GlobalStoreAndRef a lot harder to use.

Fixes #431.

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.05%. Comparing base (e3c1ff0) to head (69077c5).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #904      +/-   ##
==========================================
- Coverage   94.09%   94.05%   -0.04%     
==========================================
  Files          53       53              
  Lines        7805     7722      -83     
==========================================
- Hits         7344     7263      -81     
+ Misses        461      459       -2     

Files	Coverage Δ
include/clad/Differentiator/ReverseModeVisitor.h	97.84% <ø> (-0.05%)	⬇️
lib/Differentiator/ReverseModeVisitor.cpp	97.16% <100.00%> (-0.02%)	⬇️

Files	Coverage Δ
include/clad/Differentiator/ReverseModeVisitor.h	97.84% <ø> (-0.05%)	⬇️
lib/Differentiator/ReverseModeVisitor.cpp	97.16% <100.00%> (-0.02%)	⬇️

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[parth-07]

This looks amazing.
I will review it in-depth soon.

[vgvassilev]

This looks good to me. Let's wait for @parth-07's review here.

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[parth-07]

This pull-request has simplified a lot of complicated codes. Looks good to me.