[MS ABI] Give linkonce_odr, instead of external_linkage, to certain kinds of static data members

Out-of-line definitions of static data members which have an inline
initializer must get GVA_DiscardableODR linkage instead of
GVA_StrongExternal linkage.

MSVC 2013's behavior is different with respect to this and would cause
link errors if one TU provided a definition while another did not.
MSVC 2015 fixed this bug, making this OK.  Note that the 2015 behavior
is always compatible with 2013: it never produces a strong definition.

This essentially reverts r237787.

llvm-svn: 250757