[Preamble] Empty preamble is not an error.

Summary:
Empty preamble is valid for source file which doesn't have any
preprocessor and #includes.

This patch makes clang treat an empty preamble as a normal preamble.

Check: ninja check-clang

A testcase is added in https://reviews.llvm.org/D50627.

Reviewers: ilya-biryukov

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D50628

llvm-svn: 340029