gap resizing using solc v0.5.x

Related to #698

pragma solidity 0.5.8;

contract V1 {
    address public a;

    /**
     * @dev This empty reserved space is put in place to allow future versions to add new
     * variables without shifting down storage in the inheritance chain.
     * See https://docs.openzeppelin.com/contracts/4.x/upgradeable#storage_gaps
     */
    uint256[50] private __gap;
}

pragma solidity 0.5.8;

contract V2 {
    address public a;
    address public b;
    /**
     * @dev This empty reserved space is put in place to allow future versions to add new
     * variables without shifting down storage in the inheritance chain.
     * See https://docs.openzeppelin.com/contracts/4.x/upgradeable#storage_gaps
     */
    uint256[49] private __gap;
}

Test:

       const v1 = await deployTransparentProxy("V1", deployer, V1__factory);
       const V1 =  await ethers.getContractFactory("V1");
       const V2 =  await ethers.getContractFactory("V2");
        // forces the import of an existing proxy to be used with this plugin
        await upgrades.forceImport(v1.address, V1);
        // validates an implementation
        await upgrades.validateImplementation(V2);
        // compares the current implementation to the new implementation to check for storage layout compatibility errors
        await upgrades.validateUpgrade(v1.address, V2);

Error:
`Error: New storage layout is incompatible

contracts/V2.sol:5: Inserted b

New variables should be placed after all existing inherited variables

contracts/V2.sol:11: Upgraded __gap to an incompatible type

• Bad storage gap resize from 50 to 49
  Size decrease must match with corresponding variable inserts`

For newer solc versions, at leat > v0.7.6, works fine

Start by reproducing the V1/V2 example with solc 0.5.8, using deployTransparentProxy, forceImport, validateImplementation, and validateUpgrade as shown. Compare the result with solc versions newer than 0.7.6, then inspect the storage-layout validation path responsible for the inserted b and resized __gap errors. Done means the behavior is corrected or clearly documented with regression coverage for the demonstrated versions.