Support src.cpp and src.c in the same rules dir

[lucic71]

For C string functions, C++ defines const overloads. For example, in C, strchr is defined as:

char *strchr(const char *s, int c);

In C++, it's overloaded as:

const char *strchr(const char *s, int c);
char *strchr(char *s, int c);

Writing char *f5(const char *a0, int a1) { return strchr(a0, a1); } would result in the following error:

error: cannot initialize return object of type 'char *' with an rvalue of type 'const char *'
    6 | char *f6(const char *a0, int a1) { return strchr(a0, a1); }

For this reason, parse both src.cpp and src.c in a rules dir. src.c keeps C specific rules and src.cpp keeps C++ specific rules. The function names in the 2 functions cannot collide, cpp-rule-preprocessor checks against colliding names.

[nunoplopes]

!root.try_emplace(k, v).second

[lucic71]

Done