[ASTImporterTest] Add mandatory testing with '-fdelayed-template-parsing'

 * While running ASTImporterTests, we often forget about Windows MSVC
   buildbots which enable '-fdelayed-template-parsing' by default.
   This leads to AST import errors because templates are not parsed
   and corresponding parts of AST are not built so importer cannot import them.
   It takes both reviewing time to find such issues and post-commit time
   to fix unexpected buildbot failures. To solve this issue, we enable testing
   with '-fdelayed-template-parsing' option enabled by default in addition
   to testing with default compiler options. This allows us to catch
   the problem during development.

Differential Revision: https://reviews.llvm.org/D41444

llvm-svn: 321285