CONTRACTS: place contracts after do for do-while loops

[remi-delmas-3000]

This PR changes the syntax of do-while loop contracts from

do 
{
   body;
} while(guard)
__CPROVER_assigns(A)
__CPROVER_loop_invariant(I)
__CPROVER_decreases(D)
;

do
__CPROVER_assigns(A)
__CPROVER_loop_invariant(I)
__CPROVER_decreases(D)
{
   body;
} while(guard);

To make it more obvious that the invariant is going to be checked in the base case before entering the loop even once.

Remark: This is an internal change that will not break any existing user code since do-while loop contracts are still not officially supported.

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• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
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• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
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[qinheping]

LGTM