ARM: properly lower dllimport'ed global values

ARM: properly lower dllimport'ed global values

This completes the handling for DLL import storage symbols when lowering
instructions.  A DLL import storage symbol must have an additional load
performed prior to use.  This is applicable to variables and functions.

This is particularly important for non-function symbols as it is possible to
handle function references by emitting a thunk which performs the translation
from the unprefixed __imp_ symbol to the proper symbol (although, this is a
non-optimal lowering).  For a variable symbol, no such thunk can be
accommodated.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212431 91177308-0d34-0410-b5e6-96231b3b80d8