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:

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