[REG 6.2 -> 6.3.0] QDateTime::fromString slow at handling large input

1. Build the libfuzzer test for QDateTime::fromString, e.g.:

   ~/dev/clang-10.0.0/qt-dev_10.13-base_svg-devbld-no-openssl/qtbase/bin/qt-cmake -S ~/dev/src/qtbase/tests/libfuzzer/corelib/time/qdatetime/fromstring/ -GNinja && cmake --build . --parallel
   

2. Run the resulting binary, passing in the attached file:

   ./fromstring 38077.txt
   

   It takes too long to finish. Times I see are:

   developer build, debug, no sanitizers	9901 ms
   static, release, UBSAN	10293 ms
   static, release, ASAN	68568 ms

   The slow formats seem to be Qt::TextDate and Qt::RFC2822Date.

I admit, the file is a tad larger than usual, but why would fromString() even try to handle all of this data?

With Qt 6.2.0 and this input, fromString() finishes quickly, e.g. in 35ms with ASAN.

Google's oss-fuzz found this as issue 38077. They will publish the details on December 2nd.