Various cleanups

[StephanTLavavej]

• Improve comments citing MSDN.
• Comments: Consistently capitalize ASan (and other minor cleanups).
• ppltasks.cpp: Add preprocessor comments.
• pplerror.cpp: Adjust preprocessor logic, no functional change.
• Avoid multi-assigns in mersenne_twister_engine::seed(), no functional change.
  • This reverses the order in which we initialize/assign the local variable _Prev but that has no effect.
• ranges::iota can use contiguous_iterator.
• Improve preprocessor comments for ASan annotations.
• Avoid multi-definitions.

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[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo. Please notify me if any further changes are pushed, otherwise no action is required.

[davidmrdavid]