[codeview] Let the X86 backend tell us the VFRAME offset adjustment

Use MachineFrameInfo's OffsetAdjustment field to pass this information
from the target to CodeViewDebug.cpp. The X86 backend doesn't use it for
any other purpose.

This fixes PR38857 in the case where there is a non-aligned quantity of
CSRs and a non-aligned quantity of locals.

llvm-svn: 346062