Use dotted format of version tuple for availability diagnostics

E. g. use "10.11" instead of "10_11".

We are maintaining backward compatibility by parsing underscore-delimited version tuples but no longer keep track of the separator and using dot format for output.

Differential Revision: https://reviews.llvm.org/D46747

rdar://problem/39845032

llvm-svn: 332598
diff --git a/clang/lib/Basic/VersionTuple.cpp b/clang/lib/Basic/VersionTuple.cpp
index 9c73fd9..7a9176a 100644
--- a/clang/lib/Basic/VersionTuple.cpp
+++ b/clang/lib/Basic/VersionTuple.cpp
@@ -29,11 +29,11 @@
                                      const VersionTuple &V) {
   Out << V.getMajor();
   if (Optional<unsigned> Minor = V.getMinor())
-    Out << (V.usesUnderscores() ? '_' : '.') << *Minor;
+    Out << '.' << *Minor;
   if (Optional<unsigned> Subminor = V.getSubminor())
-    Out << (V.usesUnderscores() ? '_' : '.') << *Subminor;
+    Out << '.' << *Subminor;
   if (Optional<unsigned> Build = V.getBuild())
-    Out << (V.usesUnderscores() ? '_' : '.') << *Build;
+    Out << '.' << *Build;
   return Out;
 }