Fix outstanding clang-tidy errors. (#3654)

Recent runs of `clang-tidy` for me started showing more errors, and this
is a collection of changes to address them.

First, I've systematically applied the disabling tag to all C++ rules
under //explorer/... with `buildozer` so we don't spend time analyzing
this code or reporting errors from it. Not sure this was strictly
necessary, but it seemed like a nice consistency improvement.

Next, I disabled a buggy check for missing `default` cases in
`switch`es. It seems to get confused by the fancy conversions in our
`enum_base.h`. We don't miss much with this as the Clang compiler
warnings for `switch` catch most of our actual bugs. I also removed the
local disabling of this now that it is turned off centrally.

Lastly, I added error checking to two file descriptor manipulating calls
in the `file_test` infrastructure.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

testing/file_test/file_test_base.cpp

@@ -729,8 +729,18 @@ static auto Main(int argc, char** argv) -> int {
   // Tests might try to read from stdin. Ensure those reads fail by closing
   // stdin and reopening it as /dev/null. Note that STDIN_FILENO doesn't exist
   // on Windows, but POSIX requires it to be 0.
-  llvm::sys::Process::SafelyCloseFileDescriptor(0);
-  llvm::sys::Process::FixupStandardFileDescriptors();
+  if (std::error_code error =
+          llvm::sys::Process::SafelyCloseFileDescriptor(0)) {
+    llvm::errs() << "Unable to close standard input: " << error.message()
+                 << "\n";
+    return EXIT_FAILURE;
+  }
+  if (std::error_code error =
+          llvm::sys::Process::FixupStandardFileDescriptors()) {
+    llvm::errs() << "Unable to correct standard file descriptors: "
+                 << error.message() << "\n";
+    return EXIT_FAILURE;
+  }
 
   llvm::SmallVector<std::string> tests = GetTests();
   auto test_factory = GetFileTestFactory();