[CodingStandard] Require Unix line endings for C/C++ source and headers

Require all C/C++ source files to use Unix style line endings. This
is just formalizing an already loosely followed convention and not doing
so can cause build failure for some downstream clients of LLVM.

Author: jurahul

llvm/docs/CodingStandards.rst

@@ -1790,6 +1790,13 @@ would help to avoid running into a "dangling else" situation.
       markAsIgnored(D);
   }
 
+Use Unix style line endings for source files
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Use Unix style line endings for C/C++ source files (``.c``, ``.cpp`` and header
+files). This is required to avoid various build issue for downstream clients of
+LLVM. Test files like C/C++/HLSL test inputs can continue to use any line ending
+style.
 
 See Also
 ========