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/check/context.cpp

@@ -799,8 +799,6 @@ class TypeCompleter {
     CARBON_CHECK(xref_inst.kind() == SemIR::Builtin::Kind)
         << "TODO: Handle other kinds of inst cross-references";
 
-    // clang warns on unhandled enum values; clang-tidy is incorrect here.
-    // NOLINTNEXTLINE(bugprone-switch-missing-default-case)
     switch (xref_inst.As<SemIR::Builtin>().builtin_kind) {
       case SemIR::BuiltinKind::TypeType:
       case SemIR::BuiltinKind::Error:
@@ -919,9 +917,6 @@ class TypeCompleter {
     // dedicated file-scope instruction block where possible, or somewhere else
     // that better reflects the definition of the type, rather than wherever the
     // type happens to first be required to be complete.
-
-    // clang warns on unhandled enum values; clang-tidy is incorrect here.
-    // NOLINTNEXTLINE(bugprone-switch-missing-default-case)
     switch (inst.kind()) {
       case SemIR::AddrOf::Kind:
       case SemIR::AddrPattern::Kind: