[lld/COFF] Improve handling of the /manifestdependency: flag

If multiple /manifestdependency: flags are passed, they are
naively deduped, but after that each of them should have an
effect, instead of just the last one.

Also, /manifestdependency: flags are allowed in .drectve sections
(from `#pragma comment(linker, ...`). To make the interaction between
/manifestdependency: flags enabling manifest by default but
/manifest:no overriding this work, add an explict ManifestKind::Default
state to represent no explicit /manifest flag being passed.
To make /manifestdependency: flags from input file .drectve sections
work with /manifest:embed, delay embedded manifest emission until
after input files have been read.

Differential Revision: https://reviews.llvm.org/D108628