[SanitizerBinaryMetadata] Emit constants as ULEB128

Emit all constant integers produced by SanitizerBinaryMetadata as
ULEB128 to further reduce binary space used. Increasing the version is
not necessary given this change depends on (and will land) along with
the bump to v2.

To support this, the !pcsections metadata format is extended to allow
for per-section options, encoded in the first MD operator which must
always be a string and contain the section: "<section>!<options>".

Reviewed By: dvyukov

Differential Revision: https://reviews.llvm.org/D143484