Rename human-readable name for DW_LANG_Mips_Assembler

The Mips in DW_LANG_Mips_Assembler is a vendor name not an
architecture name and in lack of a proper generic DW_LANG_assembler,
some assemblers emit DWARF using this tag. Due to a warning I recently
introduced users will now be greeted with

  This version of LLDB has no plugin for the mipsassem language. Inspection of frame variables will be limited.

By renaming this to just "Assembler" this error message will make more sense.

Differential Revision: https://reviews.llvm.org/D101406

rdar://77214764