[formatv] Add the ability to specify a fill character when aligning.

Previously if you used fmt_align(7, Center) you would get the
output '   7   '.  It may be desirable for the user to specify
the fill character though, for example producing '---7---'.  This
patch adds that.

llvm-svn: 305449
diff --git a/llvm/unittests/Support/FormatVariadicTest.cpp b/llvm/unittests/Support/FormatVariadicTest.cpp
index 99b90b1..5387a8a 100644
--- a/llvm/unittests/Support/FormatVariadicTest.cpp
+++ b/llvm/unittests/Support/FormatVariadicTest.cpp
@@ -542,6 +542,8 @@
 
   EXPECT_EQ("  171  ",
             formatv("{0}", fmt_align(N, AlignStyle::Center, 7)).str());
+  EXPECT_EQ("--171--",
+            formatv("{0}", fmt_align(N, AlignStyle::Center, 7, '-')).str());
   EXPECT_EQ(" 171   ", formatv("{0}", fmt_pad(N, 1, 3)).str());
   EXPECT_EQ("171171171171171", formatv("{0}", fmt_repeat(N, 5)).str());