[c++20] P0515R3: Parsing support and basic AST construction for operator <=>.

[c++20] P0515R3: Parsing support and basic AST construction for operator <=>.

Adding the new enumerator forced a bunch more changes into this patch than I
would have liked. The -Wtautological-compare warning was extended to properly
check the new comparison operator, clang-format needed updating because it uses
precedence levels as weights for determining where to break lines (and several
operators increased their precedence levels with this change), thread-safety
analysis needed changes to build its own IL properly for the new operator.

All "real" semantic checking for this operator has been deferred to a future
patch. For now, we use the relational comparison rules and arbitrarily give
the builtin form of the operator a return type of 'void'.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@320707 91177308-0d34-0410-b5e6-96231b3b80d8