[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/lib/Format/ContinuationIndenter.cpp b/clang/lib/Format/ContinuationIndenter.cpp
index d28254d..d53cf80 100644
--- a/clang/lib/Format/ContinuationIndenter.cpp
+++ b/clang/lib/Format/ContinuationIndenter.cpp
@@ -1385,9 +1385,10 @@
   // 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 @@
   //     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())},