Fix the following bug submitted by Ted Kremenek:

void func() {
int xx = xx; // incorrectly diagnosed 'xx' as an undeclared identifier.
}

This smallish bug resulted in a largish fix. Here are some highlights:

- Needed to make sure ParseDeclarator is called *before* parsing any
initializer. Removed the "Init" argument to ParseDeclarator.
- Added AddInitializerToDecl() to the Action & Sema classes.
In Sema, this hook is responsible for validating the initializer and
installing it into the respective decl.
- Moved several semantic checks from ParseDeclarator() to 
FinalizeDeclaratorGroup(). Previously, this hook was only responsible for 
reversing a list. Now it plays a much larger semantic role. 

All of the above changes ended up simplifying ParseDeclarator(), which
is goodness...

llvm-svn: 41877