DWARFASTParserClang: Unify compilation unit language handling

Summary:
The function was not being consistent in how it retrieved the language
of the current compile unit. Sometimes it did so from the lldb CU
object, and sometimes from the DWARF die. This patch unifies the
handling on the latter. The reason for choosing the DWARF method is
because I'd eventually like to stop creating lldb CUs for dwarf type
units (and so this code needs to would need to work without them).

Reviewers: clayborg, JDevlieghere, aprantl

Subscribers: jdoerfert, lldb-commits

Differential Revision: https://reviews.llvm.org/D62481

llvm-svn: 361939