Place implictly declared functions at block scope

Such implicitly declared functions behave as if the enclosing block
contained the declaration extern int name() (C90, 6.3.3.2 Function calls),
thus their names should have block scope (C90, 6.1.2.1 Scope of identifiers).

This patch fixes https://bugs.llvm.org/show_bug.cgi?id=33224

Differential Revision: https://reviews.llvm.org/D33676

llvm-svn: 310616