Missing diagnostics "warning: style of line directive is a GCC extension"

[pmor13]

#13
int x;

$ clang -std=c11 -pedantic -Wall -Wextra -c

Expected:

<source>:1:2: warning: style of line directive is a GCC extension

Actual:

<nothing>

[ken-matsui]

To avoid conflicting the same work with other contributors: I'm working on this here.