Synchronise file extensions between C++ language and C/C++ preprocessor

Largely follows GCC's documentation at
https://gcc.gnu.org/onlinedocs/gcc/Overall-Options.html#index-file-name-suffix-71,
with the extension that we accept some uppercase variants in C/C++
pre-processing.

Fixes: diffblue#4358

src/ansi-c/c_preprocess.cpp

@@ -585,7 +585,8 @@ bool c_preprocess_gcc_clang(
      has_suffix(file, ".C") ||
   #endif
      has_suffix(file, ".c++") || has_suffix(file, ".C++") ||
-     has_suffix(file, ".cp") || has_suffix(file, ".CP"))
+     has_suffix(file, ".cp") || has_suffix(file, ".CP") ||
+     has_suffix(file, ".cc") || has_suffix(file, ".cxx"))
   {
     switch(config.cpp.cpp_standard)
     {

src/cpp/cpp_language.cpp

@@ -37,6 +37,7 @@ std::set<std::string> cpp_languaget::extensions() const
   s.insert("cpp");
   s.insert("CPP");
   s.insert("cc");
+  s.insert("cp");
   s.insert("c++");
   s.insert("ii");
   s.insert("cxx");