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

This reverts commit cff36bb.

This patch was causing build failures in certain configurations.

Author: boomanaiden154

llvm/include/llvm/ADT/SmallString.h

@@ -265,11 +265,6 @@ 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

@@ -244,10 +244,4 @@ 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