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>

toolchain/lower/function_context.cpp

@@ -41,8 +41,6 @@ auto FunctionContext::LowerBlock(SemIR::InstBlockId block_id) -> void {
   for (const auto& inst_id : sem_ir().inst_blocks().Get(block_id)) {
     auto inst = sem_ir().insts().Get(inst_id);
     CARBON_VLOG() << "Lowering " << inst_id << ": " << inst << "\n";
-    // clang warns on unhandled enum values; clang-tidy is incorrect here.
-    // NOLINTNEXTLINE(bugprone-switch-missing-default-case)
     switch (inst.kind()) {
 #define CARBON_SEM_IR_INST_KIND(Name)                     \
   case SemIR::Name::Kind:                                 \