Add a HsExplicitFlag to SpliceDecl, to improve Trac #4042

Add a HsExplicitFlag to SpliceDecl, to improve Trac #4042

The issue here is that

    g :: A -> A
    f
    data A = A

is treated as if you'd written $(f); that is the call of
f is a top-level Template Haskell splice.  This patch
makes sure that we *first* check the -XTemplateHaskellFlag
and bleat about a parse error if it's off.  Othewise we
get strange seeing "A is out of scope" errors.