constexpr std::string_view -> const std::string_view

This works around gcc 7.2/clang rC347417 which have a bad interaction with libstdc++'s implementation of P0426
constexpr std::string_view also emits a string_view object in .rodata that cannot be optimized out by clang (which means larger object file size)

So use good old const.

src/pipeline.cc

@@ -424,8 +424,8 @@ void LaunchStdin() {
   std::thread([]() {
     set_thread_name("stdin");
     std::string str;
+    const std::string_view kContentLength("Content-Length: ");
     while (true) {
-      constexpr std::string_view kContentLength("Content-Length: ");
       int len = 0;
       str.clear();
       while (true) {