In C++17, static constexpr members are implicitly inline, so they no
longer require an out-of-line definition.

Identified with readability-redundant-declaration.

Identified with misc-include-cleaner.

`getDebugNamesBucketAndHashCount` lures users to provide an array to
compute the bucket count using an O(n log n) sort. This is inefficient
as hash table based uniquifying is faster.

The performance issue matters less for Clang as the number of names is
relatively small. For `ld.lld --debug-names`, I plan to compute the
unique hash count as a side product of parallel entry pool computation,
and I just need a function to suggest a bucket count.

Fix uninitialized var and shift overflow.

The code suggestion was neglected when the patch landed.

llvm::dwarf::getDebugNamesBucketCount directly returns the bucket count,
via return statement, but it also returns the hash count via a
parameter. This changes the function to return them both as a std::pair,
in the return statement. It also changes the name of the function to
make it clear it returns both values.

[LLVM][DWARF]  Refactor code for generating DWARF v5 .debug_names

Refactor the code that uniques the entries and computes the bucket count
for the DWARF V5 .debug_names accelerator table.

This is followup to https://github.com/llvm/llvm-project/pull/8120.
Missed a destuctor.