[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@326996 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Format/ContinuationIndenter.cpp

@@ -1385,9 +1385,10 @@ unsigned ContinuationIndenter::reformatRawStringLiteral(
   // violate the rectangle rule and visually flows within the surrounding
   // source.
   bool ContentStartsOnNewline = Current.TokenText[OldPrefixSize] == '\n';
-  unsigned NextStartColumn = ContentStartsOnNewline
-                                 ? State.Stack.back().Indent + Style.IndentWidth
-                                 : FirstStartColumn;
+  unsigned NextStartColumn =
+      ContentStartsOnNewline
+          ? State.Stack.back().NestedBlockIndent + Style.IndentWidth
+          : FirstStartColumn;
 
   // The last start column is the column the raw string suffix starts if it is
   // put on a newline.
@@ -1399,7 +1400,7 @@ unsigned ContinuationIndenter::reformatRawStringLiteral(
   //     indent.
   unsigned LastStartColumn = Current.NewlinesBefore
                                  ? FirstStartColumn - NewPrefixSize
-                                 : State.Stack.back().Indent;
+                                 : State.Stack.back().NestedBlockIndent;
 
   std::pair<tooling::Replacements, unsigned> Fixes = internal::reformat(
       RawStringStyle, RawText, {tooling::Range(0, RawText.size())},

unittests/Format/FormatTestRawStrings.cpp

@@ -681,15 +681,14 @@ int s() {
 })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() {