clangd: objective-c headers not parsed as objective-c

it is a bit cumbersome to work with objective-c headers in qtcreator, as headers are not detected as objective-c files and even if the LANGUAGE property is set and therefore the code model does not work (i.e. detects lots of "errors')

compare attached project:

• foo.h includes <Foundation/Foundation.h>, which is not parsed as objective-c(pp), so there are errors
• foo.mm is parsed as objective-cpp

ideally the file would be parsed as objective-c(pp) under two conditions:

• if the LANGUAGE property is set to OBJC(XX)
• if the header is included in an objective-c(pp) source file: some headers seem to have different parse contexts if they are used in different targets, maybe something similar could be done for objective-c(pp) as well ...