BUG: LinearSolver is not taped in the adjoint

[connorjward]

The following code

from firedrake import *
from firedrake_adjoint import *
mesh = RectangleMesh(10, 10, 1, 1)
V = FunctionSpace(mesh, "CG", 1)
v  = TestFunction(V)
dv = TrialFunction(V)
bcs = [DirichletBC(V, Constant(0), 1)]
A = assemble(inner(grad(v), grad(dv))*dx, bcs=bcs)
x = Function(V).vector()
b = assemble(inner(Constant(1),v)*ds(2))
solver = LinearSolver(A)
solver.solve(x, b)

fails with the error message

NotImplementedError: Taping with explicit FormAssembler objects is not supported yet. Use assemble instead.

This occurs because we do not tape linear solves and so the tape is still running when we hit this line.

[nbouziani]

This is the same issue as #2835, but arising from two different API calls (solve and LinearSolver). I think the solution is to fix the existing SolveLinearSystemBlock (cf. here).