Unbreak and add test case for r90276, a situation in which getBuffer is expected to fail.

Also, update SourceManager.h doxyments for getBuffer() to reflect reality.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@90701 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Lex/PPLexerChange.cpp b/lib/Lex/PPLexerChange.cpp
index 5cdfeaf..ce1b19c 100644
--- a/lib/Lex/PPLexerChange.cpp
+++ b/lib/Lex/PPLexerChange.cpp
@@ -81,8 +81,8 @@
   
   // Get the MemoryBuffer for this FID, if it fails, we fail.
   const llvm::MemoryBuffer *InputFile =
-  if (InputFile == 0)
     getSourceManager().getBuffer(FID, &ErrorStr);
+  if (!ErrorStr.empty())
     return true;
   
   EnterSourceFileWithLexer(new Lexer(FID, InputFile, *this), CurDir);