Fix ODR violations due to abuse of LLVM_YAML_IS_(FLOW_)?SEQUENCE_VECTOR

This is a short-term fix for PR33650 aimed to get the modules build bots green again.

Remove all the places where we use the LLVM_YAML_IS_(FLOW_)?SEQUENCE_VECTOR
macros to try to locally specialize a global template for a global type. That's
not how C++ works.

Instead, we now centrally define how to format vectors of fundamental types and
of string (std::string and StringRef). We use flow formatting for the former
cases, since that's the obvious right thing to do; in the latter case, it's
less clear what the right choice is, but flow formatting is really bad for some
cases (due to very long strings), so we pick block formatting. (Many of the
cases that were using flow formatting for strings are improved by this change.)

Other than the flow -> block formatting change for some vectors of strings,
this should result in no functionality change.

Differential Revision: https://reviews.llvm.org/D34907

Corresponding updates to clang, clang-tools-extra, and lld to follow.

llvm-svn: 306878