Replace PlatformKind with PlatformType.

The PlatformKind/PlatformType enums contain the same information, which requires
them to be kept in-sync. This commit changes over to PlatformType as the sole
source of truth, which allows the removal of the redundant PlatformKind.

The majority of the changes were in LLD and TextAPI.

Reviewed By: cishida

Differential Revision: https://reviews.llvm.org/D117163