libsysprof-analyze: add API to get symbol to represent process

And include a fallback in case we never got an actual Process frame which
will contain the cmdline for the process. We need to hold onto the fallback
too so that we can keep symbols lightweight by not having to reference them
so long as the document is alive.

src/libsysprof-analyze/sysprof-process-info-private.h

@@ -31,6 +31,8 @@ typedef struct _SysprofProcessInfo
   SysprofAddressLayout  *address_layout;
   SysprofMountNamespace *mount_namespace;
   SysprofSymbolCache    *symbol_cache;
+  SysprofSymbol         *fallback_symbol;
+  SysprofSymbol         *symbol;
   int                    pid;
 } SysprofProcessInfo;