[Lex] Preambles should contain the global module fragment.

For applications like clangd, the preamble remains an important optimization
when editing a module definition. The global module fragment is a good fit for
it as it by definition contains only preprocessor directives.
Before this patch, we would terminate the preamble immediately at the "module"
keyword.

Differential Revision: https://reviews.llvm.org/D158439