[lldb][NFC] Remove misleading class_language variable in DWARFASTParserClang

There is a local 'class_language' veriable in DWARFASTParserClang which is named
as if it is related to the 'class_language' member of ParsedDWARFTypeAttributes.
However, it actually only has two possible enum values: 'ObjC' (which means the
current record is a Objective-C class) or 'Unknown' (which covers all other
cases).

This is confusing for the reader and also lead to some strange code where we
have several comparisons against the value "ObjC_plus_plus" (which is always
false).

This replaces the variable with either a const bool variable (if there are
multiple checks for that condition in a function) or a direct call to the
TypeSystemClang utility method for checking if it's a Objective-C
Object/Interface type.