Allow ".c++" as a C++ source file extension

Some projects use .c++ as an extension for C++ source files, for
example: https://github.com/capnproto/capnproto
Adding this suffix allows these source files to appear in source_sets
and makes it possible to build these projects using gn without needing
to make invasive changes to them.

Change-Id: Ie036ffc81510e79643db2674ee4b39cd5cc9691f
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/9800
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Brett Wilson <brettw@chromium.org>

src/gn/source_file.cc

diff --git a/src/gn/source_file.cc b/src/gn/source_file.cc
index 89c8cf0..52066d7 100644
--- a/src/gn/source_file.cc
+++ b/src/gn/source_file.cc
@@ -23,7 +23,8 @@
 
 SourceFile::Type GetSourceFileType(const std::string& file) {
   std::string_view extension = FindExtension(&file);
-  if (extension == "cc" || extension == "cpp" || extension == "cxx")
+  if (extension == "cc" || extension == "cpp" || extension == "cxx" ||
+      extension == "c++")
     return SourceFile::SOURCE_CPP;
   if (extension == "h" || extension == "hpp" || extension == "hxx" ||
       extension == "hh" || extension == "inc" || extension == "ipp" ||