[clang] [Serialization] Fix lazy template loading (PR #133057)

[Jonas Hahnfeld via cfe-commits]

================
@@ -367,12 +367,6 @@ bool RedeclarableTemplateDecl::loadLazySpecializationsImpl(
   if (!ExternalSource)
     return false;
 
-  // If TPL is not null, it implies that we're loading specializations for
-  // partial templates. We need to load all specializations in such cases.
-  if (TPL)
----------------
hahnjo wrote:

I tried to reproduce the problem, but it's not enough information about the setup yet. First, https://github.com/abseil/abseil-cpp/ doesn't have a `modulemap`, so I created one myself:
```
module absl {
  module string {
    module string_view {
      header "install/include/absl/strings/string_view.h"
      export *
    }
    module str_split {
      header "install/include/absl/strings/str_split.h"
      export *
    }
  }
}
```
Where do you get this from in your case and how do you compile the library + user code? The small snippet I tried was:
```c++
#include <absl/strings/string_view.h>
#include <absl/strings/str_split.h>

void test() {
  std::vector<absl::string_view> views;
  std::vector<absl::string_view>& x = views;
  x = absl::StrSplit("abc", 'y');
}
```

> Unfortunately the error in Google3 doesn't go away after reverting https://github.com/llvm/llvm-project/commit/48f50c7385d79da320cdf52d3aabc83cc403ff9c.

Ok, was worth a shot. Is it possible to figure out which of the four commits causes the problems? Then at least we could get some information what is causing it.

https://github.com/llvm/llvm-project/pull/133057