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

[Petr Hosek via cfe-commits]

================
@@ -109,6 +109,13 @@ class ToolChain {
     UNW_Libgcc
   };
 
+  enum CStdLibType {
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petrhosek wrote:

```suggestion
  enum CStdlibType {
```
For consistency with `CXXStdlibType`.

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