Use Sema's PrintingPolicy when diagnosing DeclSpecs.

Sema overrides ASTContext's policy on the first emitted diagnostic
(doesn't matter if it's ignored or not). This means changing the order
of diagnostic emission in Sema suddenly changes the text of diagnostic
emitted from the parser.

In the test case -Wmissing-prototypes (ignored) was the culprit, use
'int main' to suppress that warning so we see when this regresses.
Also move it into Sema/ as it's not testing any C++.

llvm-svn: 232039