[ClangFormat] Only insert #include into the #include block in the beginning of the file.

Summary:
This avoid inserting #include into:
- raw string literals containing #include.
- #if block.
- Special #include among declarations (e.g. functions).

Reviewers: djasper

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D26909

llvm-svn: 288493