Malcolm: 6 usability improvements in GCC 15

Over on the Red Hat Developer site, David Malcolm has an article
about improvements in GCC 15, specifically focusing on the diagnostic
information that the compiler emits. This includes ASCII art with a '⚠️'
warning emoji to display the execution path when it detects a problem (like
an infinite loop in one of his examples), better C++ template errors,
machine-readable diagnostics using Static
Analysis Results Interchange Format (SARIF), better messages regarding
C23 compatibility since that is the default C version for GCC 15, and more.
Since the changes are focused on messages, there is the inevitable color-scheme update as well:

GCC will use color when emitting its text messages on stderr at a suitably modern terminal, using a few colors that seem to work well in a number of different terminal themes—but the exact rules for choosing which color to use for each aspect of the output have been rather arbitrary.

For GCC 15, I've gone through C and C++'s errors, looking for places where two different things in the source are being contrasted, such as type mismatches. These diagnostics now use color to visually highlight and distinguish the differences.