Merge #19366: tests: Provide main(...) function in fuzzer. Allow building uninstrumented harnesses with --enable-fuzz.

1087807b2bc56b9c7e7a5471c83f6ecfae79b048 tests: Provide main(...) function in fuzzer (practicalswift)

Pull request description:

  Provide `main(...)` function in fuzzer. Allow building uninstrumented harnesses with only `--enable-fuzz`.

  This PR restores the behaviour to how things worked prior to #18008. #18008 worked around an macOS specific issue but did it in a way which unnecessarily affected platforms not in need of the workaround :)

  Before this patch:

  ```
  # Build uninstrumented fuzzing harness (no libFuzzer/AFL/other-fuzzer-instrumentation)
  $ ./configure --enable-fuzz
  $ make
    CXXLD    test/fuzz/span
  /usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu/Scrt1.o: In function `_start':
  (.text+0x20): undefined reference to `main'
  collect2: error: ld returned 1 exit status
  Makefile:7244: recipe for target 'test/fuzz/span' failed
  make[2]: *** [test/fuzz/span] Error 1
  make[2]: *** Waiting for unfinished jobs....
  $
  ```

  After this patch:

  ```
  # Build uninstrumented fuzzing harness (no libFuzzer/AFL/other-fuzzer-instrumentation)
  $ ./configure --enable-fuzz
  $ make
  $ echo foo | src/test/fuzz/span
  $
  ```

  The examples above show the change in non-macOS functionality. macOS functionality is unaffected by this patch.

ACKs for top commit:
  MarcoFalke:
    ACK 1087807b2bc56b9c7e7a5471c83f6ecfae79b048

Tree-SHA512: 9c16ea32ffd378057c4fae9d9124636d11e3769374d340f68a1b761b9e3e3b8a33579e60425293c96b8911405d8b96ac3ed378e669ea4c47836af06892aca73d

0 files changed, 0 insertions, 0 deletions