[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