Replace incorrect usage of isInvalidDecl with intended setInvalidDecl

This typo would cause an attempt to multiversion 'main' to issue an
error, but not mark the function as invalid.  This patch fixes it.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@325716 91177308-0d34-0410-b5e6-96231b3b80d8