Replace fs::absolute calls with AbsPathJoin calls

This adds better test coverage and will make it easier in #20744 to remove our dependency on the two-argument boost::filesystem::absolute() function which does not have a direct equivalent in C++17.

src/fs.cpp

@@ -31,6 +31,12 @@ FILE *fopen(const fs::path& p, const char *mode)
 #endif
 }
 
+fs::path AbsPathJoin(const fs::path& base, const fs::path& path)
+{
+    assert(base.is_absolute());
+    return fs::absolute(path, base);
+}
+
 #ifndef WIN32
 
 static std::string GetErrorReason()