Switch from llvm::StringRef to char* for Toggle name to prevent static initializers

[chrdavis]

Small change to switch from llvm::StringRef to char* to prevent incurring a large number of static initializers. See dump of static initializers before and after this change from a release build of dxcompiler.dll.

static_initializers_after.txt
static_initializers_before.txt

Fixes #6896

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[llvm-beanz]

I think this is the wrong approach. In modern LLVM StringRef's constructor is a constant expression, so it doesn't have a global initializer.

We should probably fix the problem that way rather than giving up the safety that StringRef provides over unbound character pointers.

[llvm-beanz]

This change should have a potentially larger impact on static initialization without any loss of safety:

diff --git a/include/llvm/ADT/StringRef.h b/include/llvm/ADT/StringRef.h
index 957707f427c..911a7b6bc9b 100644
--- a/include/llvm/ADT/StringRef.h
+++ b/include/llvm/ADT/StringRef.h
@@ -74,7 +74,7 @@ namespace llvm {
       }
 
     /// Construct a string ref from a pointer and length.
-    /*implicit*/ StringRef(const char *data, size_t length)
+    /*implicit*/ constexpr StringRef(const char *data, size_t length)
       : Data(data), Length(length) {
         assert((data || length == 0) &&
         "StringRef cannot be built from a NULL argument with non-null length");

[bogner]

Upstream LLVM has StringLiteral which would be appropriate here, but it looks like that was added in late 2016 and isn't available in DXC. Using a constexpr StringRef should probably do the trick though.