<rdar://problem/13560075> Teach name lookup for builtin names to find hidden declarations.

Normal name lookup ignores any hidden declarations. When name lookup
for builtin declarations fails, we just synthesize a new
declaration at the point of use. With modules, this could lead to
multiple declarations of the same builtin, if one came from a (hidden)
submodule that was later made visible. Teach name lookup to always
find builtin names, so we don't create these redundant declarations in
the first place.

llvm-svn: 178711