[YAML] fix output incorrect format for block scalar string (#132897)

After outputting block scalar string, the indent will be wrong.
This patch fixes Padding after block scalar string to ensure the correct
format of yaml.

The new added ut will fail in main.
```diff
@@ -3,4 +3,4 @@
     Just a block
     scalar doc
-scalar:          a
+  scalar:          a
 ...\n
```

1 files changed, 1 insertions, 1 deletions

diff --git a/llvm/unittests/Support/YAMLIOTest.cpp b/llvm/unittests/Support/YAMLIOTest.cpp
index f03563c..080f2b96 100644
--- a/llvm/unittests/Support/YAMLIOTest.cpp
+++ b/llvm/unittests/Support/YAMLIOTest.cpp
@@ -1298,7 +1298,7 @@ TEST(YAMLIO, TestScalarAfterBlockScalar) {
 - block_scalac:     |
     AA
     BB
-scalar:          a
+  scalar:          a
 ...
 )");
 }