Make __has_extension(assume_nonnull) always true.

llvm-svn: 240969
diff --git a/clang/lib/Lex/PPMacroExpansion.cpp b/clang/lib/Lex/PPMacroExpansion.cpp
index ea0af92..3855c80 100644
--- a/clang/lib/Lex/PPMacroExpansion.cpp
+++ b/clang/lib/Lex/PPMacroExpansion.cpp
@@ -1226,6 +1226,7 @@
   // Because we inherit the feature list from HasFeature, this string switch
   // must be less restrictive than HasFeature's.
   return llvm::StringSwitch<bool>(Extension)
+           .Case("assume_nonnull", true)
            .Case("nullability", true)
            // C11 features supported by other languages as extensions.
            .Case("c_alignas", true)