Support for DWARF-5 C++ language tags.

This patch provides support for DW_LANG_C_plus_plus_11,
DW_LANG_C_plus_plus_14 tags in the Clang C++ frontend.

Patch by Sourabh Singh Tomar!
Differential Revision: https://reviews.llvm.org/D67613

Reapplies r372663 after adapting a failing test in the LLDB testsuite.

llvm-svn: 372681