[WebAssembly] Fix validation of start function

The check for valid start function was inverted.  Added a new
test in test/Object to check this case and fixed the existing
tests in for ObjectYAML.

Differential Revision: https://reviews.llvm.org/D32986

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@302560 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed