asyncdispatch+stackTraceOverride: fix premature collection (#18039) [backport:1.2]

Copying StackTraceEntry instances when nimStackTraceOverride is defined
breaks the link between a cstring field that's supposed to point at
another string field in the same object.

Sometimes, the original object is garbage collected, that memory region
reused for storing other strings, so when the StackTraceEntry copy tries
to use its cstring pointer to construct a traceback message, it accesses
unrelated strings.

This only happens for async tracebacks and this patch prevents that by
making sure we only use the string fields when nimStackTraceOverride is
defined.

Async tracebacks also beautified slightly by getting rid of an extra line
that was supposed to be commented out, along with the corresponding debugging output.

There's also a micro-optimisation to avoid concatenating two strings just
to get their combined length.

(cherry picked from commit a1c82c39af)

tests/async/tasync_traceback.nim

@@ -86,7 +86,7 @@ Async traceback:
     asyncfutures\.nim\(\d+?\)\s+?read
   \]#
 Exception message: b failure
-Exception type:
+
 
 bar failure
 Async traceback:
@@ -114,7 +114,7 @@ Async traceback:
     asyncfutures\.nim\(\d+?\)\s+?read
   \]#
 Exception message: bar failure
-Exception type:
+
 """
 
 # TODO: is asyncmacro good enough location for fooIter traceback/debugging? just put the callsite info for all?