Revert "DebugInfo: Add/support new DW_LANG codes for recent C and C++ versions"

Some buildbots are failing in Clang and LLDB tests. (I guess the LLDB
failure is due to the explicit C language tests in DwarfUnit.cpp that
need to be updated - not sure what the Clang failures are about, they
seem to be still emitting C99 when we're expecting C11 and I checked
those tests pass... maybe systems with a different C language version
default?)

This reverts commit 3c312e48.