Speed up compilation of ASTImporter

Avoid recursively instantiating importSeq. Use initializer list
expansion to stamp out a single instantiation of std::tuple of the
deduced sequence of types, and thread the error around that tuple type.
Avoids needlessly instantiating std::tuple N-1 times.

new time to compile: 0m25.985s
old time to compile: 0m35.563s

new obj size: 10,000kb
old obj size: 12,332kb

I found the slow TU by looking at ClangBuildAnalyzer results, and looked
at -ftime-trace for the file in chrome://tracing to find this.

Tested with: clang-cl, MSVC, and GCC.

Reviewed By: martong

Differential Revision: https://reviews.llvm.org/D73667