[NFC][LLDB] Using namespace llvm in EmulateInstructionRISCV

The `EmulateInstructionRISCV` uses a lot of types and functions in `llvm` and `lldb`, this change is to make the code look cleaner.

PS: This patch should be merged before D140032.

Reviewed By: DavidSpickett

Differential Revision: https://reviews.llvm.org/D140092