[clang][RISCV][NFC] Prevent data race in RVVType::computeType

Introduce a RVVTypeCache to hold the cache instead of using a local
static variable to maintain a cache.

Also made construct of RVVType to private, make sure that could be only
created by a cache manager.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D138429