[clang-format] Use NestedBlockIndent as a 0 column in formatted raw strings

Summary:
This makes the formatter of raw string literals use NestedBlockIndent for
determining the 0 column of the content inside. This makes the formatting use
less horizonal space and fixes a case where two newlines before and after the
raw string prefix were selected instead of a single newline after it:

Before:
```
aaaa = ffff(
    R"pb(
      key: value)pb");
```

After:
```
aaaa = ffff(R"pb(
    key: value)pb");
```

Reviewers: djasper, sammccall

Reviewed By: sammccall

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D44141

llvm-svn: 326996
diff --git a/clang/unittests/Format/FormatTestRawStrings.cpp b/clang/unittests/Format/FormatTestRawStrings.cpp
index 21bbc3f..daef6d7 100644
--- a/clang/unittests/Format/FormatTestRawStrings.cpp
+++ b/clang/unittests/Format/FormatTestRawStrings.cpp
@@ -681,15 +681,14 @@
 })test",
                    getRawStringPbStyleWithColumns(20)));
 
-  // Align the suffix with the surrounding FirstIndent if the prefix is not on
+  // Align the suffix with the surrounding indent if the prefix is not on
   // a line of its own.
   expect_eq(R"test(
 int s() {
-  auto S = PTP(
-      R"pb(
-        item_1: 1,
-        item_2: 2
-      )pb");
+  auto S = PTP(R"pb(
+    item_1: 1,
+    item_2: 2
+  )pb");
 })test",
             format(R"test(
 int s() {