Partial revert of r372681 "Support for DWARF-5 C++ language tags".
The change introduced new external linkage languages ("C++11" and "C++14") which not supported in C++.
It also changed the definition of the existing enum to use the DWARF constants. The problem is that "LinkageSpecDeclBits.Language" (the field that reserves this enum) is actually defined as 3 bits length (bitfield), which cannot contain the new DWARF constants. Defining the enum as integer literals is more appropriate for maintaining valid values.