[SanitizerBinaryMetadata] Optimize used space for features and UAR stack args

Optimize the encoding of "covered" metadata by:

 1. Reducing feature mask from 4 bytes to 1 byte (needs increase once we
    reach more than 8 features).

 2. Only emitting UAR stack args size if it is non-zero, saving 4 bytes
    in the common case.

One caveat is that the emitted metadata for function PC (offset), size,
and UAR size (if enabled) are no longer aligned to 4 bytes.

SanitizerBinaryMetadata version base is increased to 2, since the change
is backwards incompatible.

Reviewed By: dvyukov

Differential Revision: https://reviews.llvm.org/D143482