[index] Generate class & metaclass manglings for objc

Summary:
ObjC classes have two associated symbols, one for the class and one for the
metaclass.

This change overloads `CodegenNameGenerator::getAllManglings` to produce both
class and metaclass symbols.

While this function is called by `clang_Cursor_getCXXManglings`, it's only
called for CXXRecordDecl and CXXMethodDecl, and so libclang's behavior is
unchanged.

Reviewers: arphaman, abdulras, alexshap, compnerd

Reviewed By: compnerd

Subscribers: compnerd

Differential Revision: https://reviews.llvm.org/D37671

llvm-svn: 313997