[OpenCL] Use DW_LANG_OpenCL language tag for OpenCL C

Note regarding C++ for OpenCL:

   When compiling C++ for OpenCL, DW_LANG_C_plus_plus* is emitted.

   There is no DWARF language code defined for C++ for OpenCL as of yet,
   but DWARF issue 210514.1 has been raised to request one.

   In the mean time, continuing to emit DW_LANG_C_plus_plus* for C++ for
   OpenCL allows the potential to distinguish between C++ for OpenCL and
   OpenCL C in !DICompileUnit nodes, whereas using DW_LANG_OpenCL for
   C++ for OpenCL would prevent this.

   This change therefore leaves C++ for OpenCL as-is.

Reviewed By: shchenz, Anastasia

Differential Revision: https://reviews.llvm.org/D104118