In commit 8d61294c3e (PCH: Mark CMake PCH source files as -x
<lang>-header, 2020-09-04, v3.18.3~14^2) we removed the explicit `-x
objective-c++` flag. This broke cases with custom source extensions.
Restore the explicit `-x objective-c[++]` flag and put it before the
`<FLAGS>` placeholder. The latter will contain the proper `-x
objective-c[++]-header` value and will override the `-x objective-c[++]`
value set before.
Fixes: #21234
Add entries in Modules and Modules/Platform to support
Objective-C compiler determination and identification.
Add Modules to check Objective-C compiler flags, source
compilations, program checks, etc...
Use OBJC as the designator of the language, eg:
project(foo OBJC)
Add various tests for Objective-C language features. Add
tests to preserve C++ handling of .m and .mm files when
OBJC is not a configured language.
Co-Authored-By: Cristian Adam <cristian.adam@gmail.com>
