Fix ldc-developers#3490 avoid new allocation of StringExp data in toir.

Author: looked-at-me

dmd/expression.h

@@ -390,6 +390,11 @@ class StringExp : public Expression
         assert(sz == 1);
         return {len, static_cast<const char *>(string)};
     }
+
+    DArray<const unsigned char> peekData() const
+    {
+        return {len * sz, static_cast<const unsigned char *>(string)};
+    }
 #endif
     size_t numberOfCodeUnits(int tynto = 0) const;
     void writeTo(void* dest, bool zero, int tyto = 0) const;

gen/toir.cpp

@@ -389,7 +389,8 @@ class ToElemVisitor : public Visitor {
     llvm::StringMap<llvm::GlobalVariable *> *stringLiteralCache =
         stringLiteralCacheForType(cty);
 
-    llvm::StringRef key(e->toChars());
+    DArray<const unsigned char> keyData = e->peekData();
+    llvm::StringRef key(static_cast<const char*>(keyData.ptr), keyData.length);
     llvm::GlobalVariable *gvar;
 
     auto iter = stringLiteralCache->find(key);