VASM part 8: add Vloc

Summary: Vloc is like PhysLoc; its the holder for the virtual registers we
assign to an SSATmp. Helpers taking Vout generally need to take Vreg
and Vloc parameters instead of PhysReg and PhysLoc, to prepare for
the next diff, where code generation uses virtual registers for
nearly everything.

Reviewed By: @alexmalyshev

Differential Revision: D1460430
typechecker: Fix column numbers in errors.

Reviewed By: @pikatchu

Differential Revision: D1512788
lexer: Remove dead code.

Summary: While trying to grok the codebase, I realized that this function was never getting called.

Reviewed By: @elgenie

Differential Revision: D1499849