Fix a bug reduced from a crash when trying to use modules with libc++. We check
the linkage of functions and variables while merging declarations from modules,
and we don't necessarily have enough of the rest of the AST loaded at that
point to allow us to compute linkage, so serialize it instead.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174943 91177308-0d34-0410-b5e6-96231b3b80d8