[Frontend] Allow to use PrecompiledPreamble without calling CanReuse

Summary:
The new method 'OverridePreamble' allows to override the preamble of
any source file without checking if preamble bounds or dependencies
were changed.

This is used for completion in clangd.

Reviewers: bkramer, sammccall

Reviewed By: sammccall

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D41990

llvm-svn: 322853