Add ValueTask support comprehensively to our analyzers

[AArnott]

At least some of our analyzers specially recognize Task and Task<T> but do not recognize ValueTask and ValueTask<T> and as a result, deadlocks and other anti-patterns can creep in undetected.

We should audit all our analyzers for gaps and fill them in.

[AArnott]

Audited all analyzers for ValueTask support. Most already handle ValueTask correctly (via AsyncMethodBuilderAttribute detection, GetAwaiter-based awaitable-type discovery, or explicit checks). The only gap found was a misleading doc comment on VSTHRD114 claiming ValueTask support it never needed (ValueTask is a value type, so
eturn null;\ is a compile error and can't reach that analyzer anyway). Fixed in #1675.