ARM: Rename ARMMCExpr::VK_ to ARM::S_
Prepare for removing ARMMCExpr. Adopt the new naming convention (S_
instead of VK_; the relocation specifier was previously named
`VariantKind`)) used by most other targets.
Make ARMMCAsmInfo.h include ARMMCExpr.h and change .cpp files to include
ARMMCAsmInfo.h. We will eventually remove ARMMCExpr.h.
