Handle delayed-template-parsing functions imported into a non-dtp TU

Summary:
DelayedTemplateParsing is marked as BENIGN_LANGOPT, so we are allowed to
use a delayed template in a non-delayed TU.
(This is clangd's default configuration on windows: delayed-template-parsing
is on for the preamble and forced off for the current file)

However today clang fails to parse implicit instantiations in a non-dtp
TU of templates defined in a dtp PCH file (and presumably module?).
In this case the delayed parser is not registered, so the function is
simply marked "delayed" again. We then hit an assert:
end of TU template instantiation should not create more late-parsed templates

Reviewers: rsmith

Subscribers: ilya-biryukov, usaxena95, cfe-commits, kadircet

Tags: #clang

Differential Revision: https://reviews.llvm.org/D81474