AST: Properly calculate the linkage of a IndirectFieldDecl

getLVForNamespaceScopeDecl believed that it wasn't possible for it to
ever see an IndirectFieldDecl.  However, this can occur when determining
whether or not something is a redeclaration of a member of an anonymous
static union.

This fixes PR21858.
Revert the default changing behavior part of r216662 until we
can change the backend to be the same default. Leave the
modified/new testcases with the exception of the default behavior
since it increases our testing footprint.
Add a FIXME for unifying ARM target abi handling.