Fix null pointer dereferences on malformed bitcode and DXIL - #9026

Add error checks so that malformed bitcode or DXIL is rejected with a
diagnostic instead of dereferencing a null pointer:

 - BitcodeReader: check for null types and values returned by
   getTypeByID/getValueFwdRef in metadata (including the selective
   named-metadata parser used by IDxcPdbUtils and METADATA_OLD_FN_NODE,
   which also must reference a function-local value), constants and
   switch parsing, validate function return and argument types (for both
   FUNCTION and FUNCTION_OLD type records), switch condition types and
   record sizes (for both switch record formats), constant cast operands
   and cast instructions, and replace an assert on function-local
   metadata with an error.
 - AutoUpgrade: when upgrading a bitcast between pointers in different
   address spaces, go through an integer vector for pointer vectors, and
   do not upgrade casts whose source and destination shapes differ.
 - report_fatal_error_handler: tolerate a missing diagnostic handler.
 - DxilResourceBase::GetHLSLType: fail on a missing symbol or type, and
   DxilMDHelper::LoadDxilResourceBase: reject a non-constant resource
   symbol before it is used.
 - EmitWarningOrErrorOnFunction and ValidationContext::EmitFnFormatError:
   tolerate a null function.

Add Bitcode/invalid-bitcode.test with eighteen malformed bitcode
inputs, and Bitcode/upgrade-addrspace-bitcast.test and
Bitcode/switch-case-range.test with well-formed inputs that must still be
read correctly. Three inputs were found by bit-flipping a small valid
module; the rest were generated directly to reach records the writer no
longer emits, the selective metadata parser, switch and cast
instructions, and address space bitcasts. Before this change, all of the
malformed inputs except an old-format switch on a float condition, which
is silently accepted as invalid IR, crash the reader or end in an
uncaught exception with no diagnostic, as do the address space bitcasts
that should be upgraded. The tests use llvm-dis, which exits cleanly on
a read error, so add it to the check-clang dependencies.

Assisted-by: Claude Code