Debug Info: for static member variables, add AT_MIPS_linkage_name to the
definition DIE, to make old GDB happy.

We have a regression for old GDB when Clang uses DW_TAG_member to declare
static members inside a class, instead of DW_TAG_variable. This patch will fix
this regression.

rdar://problem/13291234


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176143 91177308-0d34-0410-b5e6-96231b3b80d8