Use StringRef instead of MemoryBuffer&.

This code doesn't care where the data it is processing comes from, so a
StringRef is probably the most natural interface.

llvm-svn: 215448
diff --git a/clang/lib/Frontend/FrontendActions.cpp b/clang/lib/Frontend/FrontendActions.cpp
index f3be243..5c0ef7a 100644
--- a/clang/lib/Frontend/FrontendActions.cpp
+++ b/clang/lib/Frontend/FrontendActions.cpp
@@ -686,7 +686,8 @@
   llvm::MemoryBuffer *Buffer
       = CI.getFileManager().getBufferForFile(getCurrentFile());
   if (Buffer) {
-    unsigned Preamble = Lexer::ComputePreamble(*Buffer, CI.getLangOpts()).first;
+    unsigned Preamble =
+        Lexer::ComputePreamble(Buffer->getBuffer(), CI.getLangOpts()).first;
     llvm::outs().write(Buffer->getBufferStart(), Preamble);
     delete Buffer;
   }