[clang] [Clang] [Sema] Fix crash instantiating a member variable template partial specialization when the enclosing class template is deserialized (PR #202958)

[Shengxin Pei via cfe-commits]

================
@@ -0,0 +1,31 @@
+// Header used to build a precompiled header that contains compiler errors.
+//
+// The unresolved #include makes this a "PCH built with compiler errors"
+// (clangd always builds preambles with -fallow-pch-with-compiler-errors, and an
+// unresolved include is a routine state in interactive editing). The class
+// template below is a reduction of libstdc++ 15's std::expected, whose private
+// member variable template __cons_from_expected has a partial specialization.
+// See GH202956.
----------------
TPPPP72 wrote:

And these comments.

https://github.com/llvm/llvm-project/pull/202958