[llvm-cov] Use std::optional in CodeCoverage.cpp (NFC)

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

llvm/tools/llvm-cov/CodeCoverage.cpp

@@ -41,6 +41,7 @@
 
 #include <functional>
 #include <map>
+#include <optional>
 #include <system_error>
 
 using namespace llvm;
@@ -157,7 +158,7 @@ class CodeCoverageTool {
 
   /// The coverage data path to be remapped from, and the source path to be
   /// remapped to, when using -path-equivalence.
-  Optional<std::pair<std::string, std::string>> PathRemapping;
+  std::optional<std::pair<std::string, std::string>> PathRemapping;
 
   /// File status cache used when finding the same file.
   StringMap<Optional<sys::fs::file_status>> FileStatusCache;