[ADT] Add std::string_view conversion to SmallString (#83397)

This patch adds a std::string_view conversion to SmallString which we've
recently found a use for downstream in a project that is using c++
standard library IO.

Author: boomanaiden154

llvm/include/llvm/ADT/SmallString.h

@@ -265,6 +265,11 @@ class SmallString : public SmallVector<char, InternalLen> {
   /// Implicit conversion to StringRef.
   operator StringRef() const { return str(); }
 
+  /// Implicit conversion to std::string_view.
+  operator std::string_view() const {
+    return std::string_view(this->data(), this->size());
+  }
+
   explicit operator std::string() const {
     return std::string(this->data(), this->size());
   }

llvm/unittests/ADT/SmallStringTest.cpp

@@ -244,4 +244,10 @@ TEST_F(SmallStringTest, GTestPrinter) {
   EXPECT_EQ(R"("foo")", ::testing::PrintToString(ErasedSmallString));
 }
 
+TEST_F(SmallStringTest, StringView) {
+  theString = "hello from std::string_view";
+  EXPECT_EQ("hello from std::string_view",
+            static_cast<std::string_view>(theString));
+}
+
 } // namespace