[llvm-undname] Flush output before demangling.

[llvm-undname] Flush output before demangling.

If an error occurs and we write it to stderr, it could appear
before we wrote the mangled name which we're undecorating.
By flushing stdout first, we ensure that the messages are always
sequenced in the correct order.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@337645 91177308-0d34-0410-b5e6-96231b3b80d8