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/parse/parse.cpp

@@ -36,8 +36,6 @@ auto Parse(Lex::TokenizedBuffer& tokens, DiagnosticConsumer& consumer,
   context.PushState(State::DeclScopeLoop);
 
   while (!context.state_stack().empty()) {
-    // clang warns on unhandled enum values; clang-tidy is incorrect here.
-    // NOLINTNEXTLINE(bugprone-switch-missing-default-case)
     switch (context.state_stack().back().state) {
 #define CARBON_PARSE_STATE(Name) \
   case State::Name:              \