[clang] [clang] Support --sysroot= for ${arch}-windows-msvc targets (PR #96417)

[via cfe-commits]

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@@ -95,18 +95,21 @@ class ToolChain {
 
   enum CXXStdlibType {
     CST_Libcxx,
-    CST_Libstdcxx
+    CST_Libstdcxx,
+    CST_Stl,
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zmodem wrote:

I think most C++ developers understand STL as a generic term for the standard library rather than for Microsoft's implementation specifically.

`-stdlib=stl` could be confusing; `-stdlib=msstl` would be much clearer. (And the enumeration should be named to match.)

https://github.com/llvm/llvm-project/pull/96417