Support: Allow use of MemoryBufferRef with line_iterator

Split out from https://reviews.llvm.org/D66782, use `Optional<MemoryBufferRef>`
in `line_iterator` so you don't need access to a `MemoryBuffer*`.  Follow up
patches in `clang/` will leverage this.

Differential Revision: https://reviews.llvm.org/D89280

include/llvm/Support/LineIterator.h

@@ -9,8 +9,10 @@
 #ifndef LLVM_SUPPORT_LINEITERATOR_H
 #define LLVM_SUPPORT_LINEITERATOR_H
 
+#include "llvm/ADT/Optional.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/DataTypes.h"
+#include "llvm/Support/MemoryBufferRef.h"
 #include <iterator>
 
 namespace llvm {
@@ -30,7 +32,7 @@ class MemoryBuffer;
 /// Note that this iterator requires the buffer to be nul terminated.
 class line_iterator
     : public std::iterator<std::forward_iterator_tag, StringRef> {
-  const MemoryBuffer *Buffer = nullptr;
+  Optional<MemoryBufferRef> Buffer;
   char CommentMarker = '\0';
   bool SkipBlanks = true;
 
@@ -41,6 +43,10 @@ public:
   /// Default construct an "end" iterator.
   line_iterator() = default;
 
+  /// Construct a new iterator around an unowned memory buffer.
+  explicit line_iterator(const MemoryBufferRef &Buffer, bool SkipBlanks = true,
+                         char CommentMarker = '\0');
+
   /// Construct a new iterator around some memory buffer.
   explicit line_iterator(const MemoryBuffer &Buffer, bool SkipBlanks = true,
                          char CommentMarker = '\0');

lib/Support/LineIterator.cpp

@@ -33,7 +33,11 @@ static bool skipIfAtLineEnd(const char *&P) {
 
 line_iterator::line_iterator(const MemoryBuffer &Buffer, bool SkipBlanks,
                              char CommentMarker)
-    : Buffer(Buffer.getBufferSize() ? &Buffer : nullptr),
+    : line_iterator(Buffer.getMemBufferRef(), SkipBlanks, CommentMarker) {}
+
+line_iterator::line_iterator(const MemoryBufferRef &Buffer, bool SkipBlanks,
+                             char CommentMarker)
+    : Buffer(Buffer.getBufferSize() ? Optional<MemoryBufferRef>(Buffer) : None),
       CommentMarker(CommentMarker), SkipBlanks(SkipBlanks), LineNumber(1),
       CurrentLine(Buffer.getBufferSize() ? Buffer.getBufferStart() : nullptr,
                   0) {
@@ -78,7 +82,7 @@ void line_iterator::advance() {
 
   if (*Pos == '\0') {
     // We've hit the end of the buffer, reset ourselves to the end state.
-    Buffer = nullptr;
+    Buffer = None;
     CurrentLine = StringRef();
     return;
   }

unittests/Support/LineIteratorTest.cpp

@@ -39,6 +39,30 @@ TEST(LineIteratorTest, Basic) {
   EXPECT_EQ(E, I);
 }
 
+TEST(LineIteratorTest, Ref) {
+  std::unique_ptr<MemoryBuffer> Buffer = MemoryBuffer::getMemBuffer("line 1\n"
+                                                                    "line 2\n"
+                                                                    "line 3");
+
+  line_iterator I = line_iterator(Buffer->getMemBufferRef()), E;
+
+  EXPECT_FALSE(I.is_at_eof());
+  EXPECT_NE(E, I);
+
+  EXPECT_EQ("line 1", *I);
+  EXPECT_EQ(1, I.line_number());
+  ++I;
+  EXPECT_EQ("line 2", *I);
+  EXPECT_EQ(2, I.line_number());
+  ++I;
+  EXPECT_EQ("line 3", *I);
+  EXPECT_EQ(3, I.line_number());
+  ++I;
+
+  EXPECT_TRUE(I.is_at_eof());
+  EXPECT_EQ(E, I);
+}
+
 TEST(LineIteratorTest, CommentAndBlankSkipping) {
   std::unique_ptr<MemoryBuffer> Buffer(
       MemoryBuffer::getMemBuffer("line 1\n"