[Clang][RISCV] Fix RISC-V vector / SiFive intrinsic inclusion in SemaLookup

The existing code assumes that both `DeclareRISCVVBuiltins` and
`DeclareRISCVSiFiveVectorBuiltins` are set when coming into the if-statement
under SemaLookup.cpp.

This is not the case and causes issue #63571.

This patch resolves the issue.

Reviewed By: 4vtomat, kito-cheng

Differential Revision: https://reviews.llvm.org/D154050