[Frontend] Fix printing policy for AST context loaded from file

In ASTUnit::LoadFromASTFile, the context object is set up using
default-constructed LangOptions (which only later get populated). As the
language options are used in the constructor of PrintingPolicy, this
needs to be updated explicitly after the language options are available.

Patch by Johann Klähn!

Differential Revision: https://reviews.llvm.org/D35271

llvm-svn: 311787