Test "merging" of typedef types across distinct modules. At present,

the AST reader doesn't actually perform a merge, because name lookup
knows how to merge identical typedefs together.

As part of this, teach C/Objective-C name lookup to return multiple
results in all cases, rather than first digging through the attributes
to see if the value is overloadable. This way, we'll catch ambiguous
lookups in C/Objective-C.

llvm-svn: 147498