Add support for mixed objective-c and pure c++ code in same apple_library

[ivan-golub]

Background:

• It is possible to create library that will contain mix of objective-c, objective-c++, pure c and pure c++ code in vanilla Xcode.
• objc_library supports all languages above as well
• Current apple_library adds .m, .mm, .c sources as well as .hm .hh, .hpp headers headers but excludes .cc and .cpp which causes compilation error if library has pure c++ code

Changes:

• Add c++ source code (.cc and .cpp) files in srcs of apple_library
• Add test project that mixes plain objective-c and pure c++

Test Plan:

• Run bazel test //tests/ios/frameworks/objcpp:ObjCppMixedLibTests

Risks:

• Logic of https://github.com/bazel-ios/rules_ios/blob/master/rules/library.bzl#L866 is not entirely clear from comment. If enable it, then lib containing both c++ and objective-c will create two libraries, one with objective-c sources and objective-c + c++ headers (likely not compilable due to missing c++ implementations) and cc library that contains all objective-c private headers and c++ implementations. cc_library idea needs clarification and either should be removed and this PR merged or revived in form that will allow tests from this PR to pass

[nkoroste]

@jerrymarino fyi

[jerrymarino]

Thanks for the PR and adding the example!

I'd be in favor to re-enable https://github.com/bazel-ios/rules_ios/pull/304/files#diff-23068963449a2bf4d162d813a4dda51236b9d044d1442e8a4bb4ad7680473478L867 to set the language level copts at the lib level

[jerrymarino]

I wonder what the the tradeoffs of this line and https://github.com/bazel-ios/rules_ios/blob/master/rules/library.bzl#L870 are ?

I don't know why https://github.com/bazel-ios/rules_ios/blob/master/rules/library.bzl#L870 was disabled, one of the problems with putting the sources in the same library is there is no way to specify the C++ specific copts at the library level.

[jerrymarino]

Also Cocoapods, it compiles C++ sources as a separate objc_library maybe we can just apply similar logic in that line to make the logic more close to that.

[mattrobmattrob]

Closing since this should be handled in #550. Reopen #537 if it doesn't!