[LLVM][TableGen][DecoderEmitter] Add wrapper struct for bit_value_t (llvm#146248)

Add a convenience wrapper struct for the `bit_value_t` enum type to host
various constructors, query, and printing support. Also refactor related
code in several places. In `getBitsField`, use `llvm::append_range` and
`SmallVector::append()` and eliminate manual loops. Eliminate
`emitNameWithID` and instead use the `operator <<` that does the same
thing as this function. Have `BitValue::getValue()` (replacement for
`Value`) return std::optional<> instead of -1 for unset bits. Terminate
with a fatal error when a decoding conflict is encountered.

Author: jurahul

llvm/test/TableGen/FixedLenDecoderEmitter/conflict.td

@@ -1,4 +1,4 @@
-// RUN: llvm-tblgen -gen-disassembler -I %p/../../../include %s -o - 2>%t
+// RUN: not llvm-tblgen -gen-disassembler -I %p/../../../include %s -o - 2>%t
 // RUN: FileCheck %s < %t
 
 include "llvm/Target/Target.td"