libclang: factor out the frequent pattern static_cast<ASTUnit *>(TU->TUData)
into a getter cxtu::getASTUnit(TU)

llvm-svn: 173585
diff --git a/clang/tools/libclang/CIndexDiagnostic.cpp b/clang/tools/libclang/CIndexDiagnostic.cpp
index 6b0a191..afccbb8 100644
--- a/clang/tools/libclang/CIndexDiagnostic.cpp
+++ b/clang/tools/libclang/CIndexDiagnostic.cpp
@@ -158,7 +158,7 @@
 
 CXDiagnosticSetImpl *cxdiag::lazyCreateDiags(CXTranslationUnit TU,
                                              bool checkIfChanged) {
-  ASTUnit *AU = static_cast<ASTUnit *>(TU->TUData);
+  ASTUnit *AU = cxtu::getASTUnit(TU);
 
   if (TU->Diagnostics && checkIfChanged) {
     // In normal use, ASTUnit's diagnostics should not change unless we reparse.