Add a comment about when we can remove this construct; NFC.

Author: AaronBallman

llvm/include/llvm/Support/Compiler.h

@@ -95,7 +95,8 @@
 /// Does the compiler support ref-qualifiers for *this?
 ///
 /// Sadly, this is separate from just rvalue reference support because GCC
-/// and MSVC implemented this later than everything else.
+/// and MSVC implemented this later than everything else. This appears to be
+/// corrected in MSVC 2019 but not MSVC 2017.
 #if __has_feature(cxx_rvalue_references) || LLVM_GNUC_PREREQ(4, 8, 1)
 #define LLVM_HAS_RVALUE_REFERENCE_THIS 1
 #else