Pull in r258110 from upstream clang trunk, by Faisal Vali:

Fix PR26134: When substituting into default template arguments, keep
  CurContext unchanged.

  Or, do not set Sema's CurContext to the template declaration's when
  substituting into default template arguments of said template
  declaration.
  If we do push the template declaration context on to Sema, and the
  template declaration is at namespace scope, Sema can get confused and
  try and do odr analysis when substituting into default template
  arguments, even though the substitution could be occurring within a
  dependent context.
  I'm not sure why this was being done, perhaps there was concern that
  if a default template argument referred to a previous template
  parameter, it might not be found during substitution - but all
  regression tests pass, and I can't craft a test that would cause it
  to fails (if some one does, please inform me, and i'll craft a
  different fix for the PR).

  This patch removes a single line of code, but unfortunately adds more
  than it removes, because of the tests.  Some day I still hope to
  commit a patch that removes far more lines than it adds, while
  leaving clang better for it ;)

  Sorry that r253590 ("Change the expression evaluation context from
  Unevaluated to ConstantEvaluated while substituting into non-type
  template argument defaults") caused the PR!

This fix will be merged to the upstream release_38 branch soon, but we
need it now, to fix a failure in the databases/sfcgal port.

contrib/llvm/tools/clang/lib/Sema/SemaTemplate.cpp

@@ -3281,7 +3281,6 @@ SubstDefaultTemplateArgument(Sema &SemaRef,
   for (unsigned i = 0, e = Param->getDepth(); i != e; ++i)
     TemplateArgLists.addOuterTemplateArguments(None);
 
-  Sema::ContextRAII SavedContext(SemaRef, Template->getDeclContext());
   EnterExpressionEvaluationContext ConstantEvaluated(SemaRef,
                                                      Sema::ConstantEvaluated);
   return SemaRef.SubstExpr(Param->getDefaultArgument(), TemplateArgLists);