[RISCV] Imply extensions in RISCVTargetInfo::initFeatureMap

Under ASTContext, clang only copies the features from the options with
Target->initFeatureMap, and no implications is done there. This makes
clang_cc1 fail to imply into `zve32x` for the vector extension, and test
cases will have to add ` -target-feature +experimental-zve32x` in order
to work.

This patch fixes it.

Reviewed By: kito-cheng

Differential Revision: https://reviews.llvm.org/D113336