When we see a reference to a struct, class, or union like "struct X"

that is neither a definition nor a forward declaration and where X has
not yet been declared as a tag, introduce a declaration
into the appropriate scope (which is likely *not* to be the current
scope). The rules for the placement of the declaration differ slightly
in C and C++, so we implement both and test the various corner
cases. This implementation isn't 100% correct due to some lingering
issues with the function prototype scope (for a function parameter
list) not being the same scope as the scope of the function
definition. Testcase is FIXME'd; this probably isn't an important issue.

Addresses <rdar://problem/6484805>.

llvm-svn: 62014