GitHub: Incorrect repository language detection

[StephanTLavavej]

Our code is being detected as 57.4% C++, 33.4% Objective-C, 7.3% C, and 1.9% CMake. There are several issues here:

1. Extensionless headers like stl/inc/vector are indexed and searchable, but aren't being detected as any language.

2. stl/inc/xcharconv_ryu_tables.h is being detected as Objective-C, but it's C++.

3. stl/src/nlsdownlevel.h is being detected as Objective-C. This is C++ (included by stl/src/winapinls.cpp only), but it uses C idioms (self-typedefs). We should probably fuse nlsdownlevel.h into winapinls.cpp and remove the C idioms.

4. Several files like stl/inc/xcharconv.h are being detected as C when they're C++. A very small number of our files are intended to be usable as both C and C++, while the rest are purely C++.

https://github.com/github/linguist#troubleshooting explains how to override this language detection.