[Vt] Update TF_FOR_ALL to C++11 iterator

[Cewein]

Description of Change(s)

Update files under pxr/base/vt by replacing TF_FOR_ALL with C++11-style range-based for each loop.

Important notes

There is an example the might be worth looking into since some of the code that not use TF_FOR_ALL could be using also C++11 iterator, specifically :

OpenUSD/pxr/base/vt/testenv/testVtCpp.cpp

Lines 88 to 90 in 1841783

	for (VtDoubleArray::const_iterator i = da.begin(); i != da.end(); ++i)
	if (*i != val++)
	die("iterator");

could just be :

for (const auto& element : da) {
    if (element != val++)
        die("iterator");
}

Open Issue(s)

This PR addresses issue #80, though many more instances still need to be resolved.

Checklist

• I have created this PR based on the dev branch

• I have followed the coding conventions

• I have added unit tests that exercise this functionality (Reference:
  testing guidelines)

• I have verified that all unit tests pass with the proposed changes

• I have submitted a signed Contributor License Agreement (Reference:
  Contributor License Agreement instructions)

[jesschimein]

Filed as internal issue #USD-10853

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[jesschimein]

/AzurePipelines run

[tallytalwar]

/AzurePipelines run

[tallytalwar]

/AzurePipelines run

[jesschimein]

/AzurePipelines run