[modules ts] Basic for module linkage.

[modules ts] Basic for module linkage.

In addition to the formal linkage rules, the Modules TS includes cases where
internal-linkage symbols within a module interface unit can be referenced from
outside the module via exported inline functions / templates. We give such
declarations "module-internal linkage", which is formally internal linkage, but
results in an externally-visible symbol.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@307434 91177308-0d34-0410-b5e6-96231b3b80d8