[C++2a] Implement operator<=>: Address bugs and post-commit review comments after r331677.

This patch addresses some mostly trivial post-commit review comments received
on r331677.

Additionally, this patch fixes an assertion in `getNarrowingKind` caused by
the use of an uninitialized value from `checkThreeWayNarrowingConversion`.

llvm-svn: 331707
diff --git a/clang/test/SemaCXX/std-compare-cxx2a.cpp b/clang/test/SemaCXX/std-compare-cxx2a.cpp
index 3aa625a..6746fb4 100644
--- a/clang/test/SemaCXX/std-compare-cxx2a.cpp
+++ b/clang/test/SemaCXX/std-compare-cxx2a.cpp
@@ -3,7 +3,7 @@
 // RUN: %clang_cc1 -triple x86_64-apple-darwin -fcxx-exceptions -fsyntax-only -pedantic -verify -Wsign-compare -std=c++2a %s
 
 void compare_not_found_test() {
-  // expected-error@+1 {{cannot deduce return type of 'operator<=>' because type partial_ordering was not found; include <compare>}}
+  // expected-error@+1 {{cannot deduce return type of 'operator<=>' because type 'std::partial_ordering' was not found; include <compare>}}
   (void)(0.0 <=> 42.123);
 }