[VE] Rename VE toolchain source files

Summary:
Rename VE.cpp and VE.h to VEToolchain.cpp and VEToolchain.h respectively
in order to avoid link warning message.  Linker warns that VE.cpp.o and
Arch/VE.cpp.o have the same name.

Reviewers: simoll, k-ishizaka

Reviewed By: simoll

Subscribers: mgorny, cfe-commits

Tags: #llvm, #ve, #clang

Differential Revision: https://reviews.llvm.org/D82968