How a diagnostic is identified (C90 3.7, C99 and C11 3.10, C90,
C99 and C11 5.1.1.3).
Diagnostics consist of all the output sent to stderr by GCC.
Whether each nonempty sequence of white-space characters other than
new-line is retained or replaced by one space character in translation
phase 3 (C90, C99 and C11 5.1.1.2).