Add heuristics for C++ macros

[jtibshirani]

This PR adds a new heuristic for C++ that checks for the preprocessor macros
__has_cpp_attribute and __cplusplus. These are commonly-used macros that check
what version of C++ is supported, and the available attributes.

Checklist:

• I am fixing a misclassified language
  • I have included a new sample for the misclassified language:
    • Sample source(s): NoDiscard.h from Swift project
    • Sample license(s): Apache v2
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

[lildude]

LGTM. Thanks.

Note: this PR will not be merged until close to when the next release is made. See here for more details.

[jtibshirani]

Thanks for the speedy review!