InstrProf: Fix a misuse of the FunctionDecl API when generating coverage

This was calling FD->hasBody(), meaning "Does the function that this
decl refers to have a body?", rather than
FD->doesThisDeclarationHaveABody(), meaning "Is this decl a
non-deleted definition?".

We might want to consider renaming these APIs :/

llvm-svn: 243360