AST: Mangle fields in anonymous structs/unions

AST: Mangle fields in anonymous structs/unions

The Itanium mangler couldn't cope with mangling an IndirectFieldDecl.
Instead, mangle the field the IndirectFieldDecl refers to.

Further, give IndirectFieldDecl no linkage just like FieldDecl.

N.B. Decl.cpp:getLVForNamespaceScopeDecl tried to calculate linkage for
data members of anonymous structs/unions.  However, this seems
impossible so turn it into an assertion.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193269 91177308-0d34-0410-b5e6-96231b3b80d8