[dwarfdump] Add missing DW_LANG_Mips_Assembler case to LanguageString()

Reviewed By: dblaikie

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5193

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217182 91177308-0d34-0410-b5e6-96231b3b80d8