Remove uses of the redundant ".reset(nullptr)" of unique_ptr, in favor of ".reset()"

It's also possible to just write "= nullptr", but there's some question
of whether that's as readable, so I leave it up to authors to pick which
they prefer for now. If we want to discuss standardizing on one or the
other, we can do that at some point in the future.

llvm-svn: 213439
diff --git a/clang/lib/Frontend/TextDiagnosticPrinter.cpp b/clang/lib/Frontend/TextDiagnosticPrinter.cpp
index 6271509..66b46b7 100644
--- a/clang/lib/Frontend/TextDiagnosticPrinter.cpp
+++ b/clang/lib/Frontend/TextDiagnosticPrinter.cpp
@@ -43,7 +43,7 @@
 }
 
 void TextDiagnosticPrinter::EndSourceFile() {
-  TextDiag.reset(nullptr);
+  TextDiag.reset();
 }
 
 /// \brief Print any diagnostic option information to a raw_ostream.