This MIME type name is used in Qt Creator  and KDevelop . A different name - text/x-objcpp-src - is used in KSyntaxHighlighting . The name text/x-objc++src is consistent with the existing MIME type names text/x-c++src and text/x-objcsrc. So KSyntaxHighlighting's name will have to be replaced later.
The Objective-C Wikipedia article and the KSyntaxHighlighting's data file  claim that *.M is one of the file extensions of Objective-C++ source files. But since macOS's file system is case-insensitive by default, *.mm is used almost universally. Thus there is no need to complicate the MIME type database with case-sensitive attributes.
Make text/x-troff-mm a subclass of text/troff to reuse its magic element. As far as I can tell, these two file formats share the same comment syntax, which is encoded in text/troff's magic.
Add Objective-C, Objective-C++, Troff, Troff MM and C tests to verify correct detection of the new MIME type and the absence of regressions.