Fixed undefined behavior in pushMappings when the stack has to resize.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@84924 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Basic/Diagnostic.cpp

@@ -230,6 +230,8 @@ Diagnostic::~Diagnostic() {
 
 
 void Diagnostic::pushMappings() {
+  // Avoids undefined behavior when the stack has to resize.
+  DiagMappingsStack.reserve(DiagMappingsStack.size() + 1);
   DiagMappingsStack.push_back(DiagMappingsStack.back());
 }