Change the DWARFExpression::Evaluate methods to take an optional

RegisterContext* - normally this is retrieved from the ExecutionContext's
StackFrame but when we need to evaluate an expression while creating
the stack frame list this can be a little tricky.

Add DW_OP_deref_size, needed for the _sigtramp FDE expression.

Add support for processing DWARF expressions in RegisterContextLLDB.

Update callers to DWARFExpression::Evaluate.

llvm-svn: 119885