[clang] [Clang] Add clang flag --cstdlib (PR #183254)

[Petr Hosek via cfe-commits]

================
@@ -194,6 +201,7 @@ class ToolChain {
   mutable std::optional<CXXStdlibType> cxxStdlibType;
   mutable std::optional<RuntimeLibType> runtimeLibType;
   mutable std::optional<UnwindLibType> unwindLibType;
+  mutable std::optional<CStdLibType> cStdLibType;
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petrhosek wrote:

```suggestion
  mutable std::optional<CStdLibType> cStdlibType;
```
For consistency with `cxxStdlibType`.

https://github.com/llvm/llvm-project/pull/183254