Ensure CheckTemplateArgumentList is always called with a non-empty CodeSynthesisContext (#202)

Fixes an internal Clang assertion failure triggered when preprocessing
the rules for `std::unique_ptr` with `--cxxflags='-stdlib=libc++'`.

Author: joaotgouveia

cpp2rust/cpp_rule_preprocessor.cpp

@@ -270,6 +270,7 @@ class Callback : public clang::ast_matchers::MatchFinder::MatchCallback {
 
         clang::DefaultArguments defaultArgs;
         clang::Sema::CheckTemplateArgumentInfo ctai;
+        clang::Sema::InstantiatingTemplate Inst(*sema_, loc_, decl);
         auto invalid = sema_->CheckTemplateArgumentList(
             decl, decl->getTemplateParameters(), loc_, targsInfo, defaultArgs,
             true, ctai);