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

[krasimir]

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");

[sammccall]

LGTM, probably want to wait for djasper's opinion.

cfe/trunk/lib/Format/ContinuationIndenter.cpp

unsigned ContinuationIndenter::reformatRawStringLiteral(
   //   - if the content starts on newline, it is one level more than the current
   //     indent, and
   //   - if the content does not start on a newline, it is the first start
   //     column.
   // These rules have the advantage that the formatted content both does not
   // 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
 
   // The last start column is the column the raw string suffix starts if it is
   // put on a newline.
   // The last start column is the intended indentation of the raw string postfix
   // if it is put on a newline. It is determined by the following rules:
   //   - if the raw string prefix starts on a newline, it is the column where
   //     that raw string prefix starts, and
   //   - if the raw string prefix does not start on a newline, it is the current
   //     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())},
       FirstStartColumn, NextStartColumn, LastStartColumn, "<stdin>",
       /*Status=*/nullptr);
 
   auto NewCode = applyAllReplacements(RawText, Fixes.first);
   tooling::Replacements NoFixes;

cfe/trunk/unittests/Format/FormatTestRawStrings.cpp

 int s() {
   auto S = PTP(
       R"pb(
       item_1: 1,
       item_2: 2)pb");
 })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(
 })test",
             format(R"test(
 int s() {
   auto S = PTP(R"pb(
       item_1: 1,
       item_2: 2
       )pb");
 })test",