CXX-3253 reintroduce VS 2015 task coverage

[eramongodb]

Resolves CXX-3253. Followup to mongodb/mongo-c-driver#1991.

[eramongodb]

Converting to draft due to discovery of overlooked EVG configuration errors. Will fix and re-request reviews when ready.

[eramongodb]

Identified compile_vars was not being passed to Compile.call() as it should be. This means the REQUIRED_CXX_STANDARD and USE_STATIC_LIBS environment variables were not being applied as they should.

While addressing this bug, took the opportunity to extend compiler coverage to better match that of the C Driver's std-matrix tasks, including coverage with /std:c++latest (which is supported on EVG distros unlike /std:clatest). Rather than separate tasks for shared and static libraries, the BUILD_SHARED_AND_STATIC_LIBS option is set to ON instead.

Both of these updates revealed several issues which are addressed by this PR, as described below.

---

Identified a few more references to C Driver hedged reads API in test code that were missed in #1351. Applied suppressions accordingly.

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The false-positive -Wmaybe-uninitialized warning for optional<T> made a return with GCC 8 and newer. Although it was meant to be suppressed by #968, the same false-positive warnings were (inconsistently) observed in other functions returning an optional<T> as well, e.g.:

uri.cpp: In member function 'std::optional<int> mongocxx::v_noabi::uri::wait_queue_timeout_ms() const':
error: '<anonymous>' may be used uninitialized in this function [-Werror=maybe-uninitialized]
         return {};
                 ^

To avoid this issue for good, changed all instances of the problematic return {}; pattern to return bsoncxx::v_noabi::stdx::nullopt; instead.

---

Extending compiler coverage to GCC 4.8 (on rhel76) revealed the following compilation error:

error: unknown option after '#pragma GCC diagnostic' kind [-Werror=pragmas]
  BSONCXX_PRIVATE_WARNINGS_DISABLE(GNU("-Wignored-attributes"));

This required adding the defined(__GNUC__) && (__GNUC__ >= 6) && !defined(__clang__) condition to avoid suppressing an unsupported warning flag for GCC versions older than GCC 6.

---

Following #1329, there should have been no more alignment-related issues in the C++ Driver. However, when removing an overlooked -Wno-aligned-new flag in the compile script, more instances of this warning were identified with Clang 13 (on rhel90), e.g.:

error: passing 4-byte aligned argument to 8-byte aligned parameter 1 of 'bson_get_data' may result in an unaligned pointer access [-Werror,-Walign-mismatch]
    return {bson_get_data(failure), failure->len};
                          ^

Further investigation revealed a common pattern for all instances of this warning: the use of auto to deduce a bson_t const*. This suggests auto is also susceptible to the "attributes are ignored" issue which affects template parameters as described in #1329. Changing all instances of such auto variables to explicit bson_t const* addressed these warnings.

[kevinAlbs]

The compile matrix improvements (easier visibility to which compiler version is tested) are very much appreciated. LGTM

[eramongodb]

Deferred attempts to resolve rhel76 task failure to #1384.

[vector-of-bool]

LGTM