Identified with modernize-use-using.

Identified with readability-redundant-typename.

- Move standalone functions and variables out of anonymous namespace and
make them static.
- Eliminate `namespace llvm {}` wrapping all code in .cpp files, and
instead use namespace qualifier to define such functions
(https://llvm.org/docs/CodingStandards.html#use-namespace-qualifiers-to-implement-previously-declared-functions)
- Add namespace for X86DisassemblerShared.h.

Tolerate setting negative values in tablegen, and store them as a
saturated uint8_t value. This will allow naive uses of the copy cost
to directly add it as a cost without considering the degenerate negative
case. The degenerate negative cases are only used in InstrEmitter / DAG
scheduling, so leave the special case processing there. There are also
fixmes about this system already there.

This is the expedient fix for an out of tree target regression
after #160084. Currently targets can set a negative copy cost to mark
copies as "impossible". However essentially all the in-tree uses only
uses this for non-allocatable condition registers. We probably should
replace the InstrEmitter/DAG scheduler uses with a more direct check
for a copyable register but that has test changes.

This makes CodeGenRegisterClass::contains fast. Use this to simplify
inferMatchingSuperRegClass.

Each of the SRSets is already sorted and unique because it is a filtered
version of RC->getMembers() which is already sorted and unique.

TableGen's RegUnitIterator is a strange contraption that iterates over a
range of registers as well as the regunits of each register. Since it is
only used in one place in a `for` loop, it is much simpler to use two
nested loops instead.

llvm::is_contained is shorter than llvm::all_of plus a lambda.