[modules-ts] Omit submodule semantics for TS modules

If a TS module name has more than one component (e.g., foo.bar) then we
erroneously activated the submodule semantics when encountering a module
declaration in the module implementation unit (e.g., 'module foo.bar;').

Reviewed By: rsmith

Differential Revision: https://reviews.llvm.org/D35678

llvm-svn: 312007