Add the file a symbol is declared in to Reflection (#6613)

* Add the file a symbol is declared in to Reflection

If we move a code-generator to depend on Reflection,
it may need to know which file something was declared in
to properly name generated files.

* Doc comments in reflection, and more precise tests

* Add --project-root flag to flatc, normalize declaraion_file to this root

* fix --project-root stuff

* posixpath

* fix scripts

* format

* rename --project-root to --bfbs-filenames

Also, make it optional, rather than defaulting to `./`, if its not
specified, then don't serialize the filenames.

* bfbs generation

* fix some tests

* uncomment a thing

* add  to project root directory conditionally

* fix

* git clang format

* Added help description and removed != nullptr

* "

* Remove accidental change to docs

* Remove accidental change to docs

* Pool strings

Co-authored-by: Casper Neo <cneo@google.com>

include/flatbuffers/util.h

@@ -468,6 +468,7 @@ std::string ConCatPathFileName(const std::string &path,
 
 // Replaces any '\\' separators with '/'
 std::string PosixPath(const char *path);
+std::string PosixPath(const std::string &path);
 
 // This function ensure a directory exists, by recursively
 // creating dirs for any parts of the path that don't exist yet.
@@ -477,6 +478,10 @@ void EnsureDirExists(const std::string &filepath);
 // Returns the input path if the absolute path couldn't be resolved.
 std::string AbsolutePath(const std::string &filepath);
 
+// Returns files relative to the --project_root path, prefixed with `//`.
+std::string RelativeToRootPath(const std::string &project,
+                               const std::string &filepath);
+
 // To and from UTF-8 unicode conversion functions
 
 // Convert a unicode code point into a UTF-8 representation by appending it