libstdc++: Add inline keyword to _M_locate

libstdc++: Add inline keyword to _M_locate

In GCC 12 there was a ~40% regression in the performance of hashmap->find.

This regression came about accidentally:

Before GCC 12 the find function was small enough that IPA would inline it even
though it wasn't marked inline.  In GCC-12 an optimization was added to perform
a linear search when the entries in the hashmap are small.

This increased the size of the function enough that IPA would no longer inline.
Inlining had two benefits:

1.  The return value is a reference. so it has to be returned and dereferenced
    even though the search loop may have already dereference it.
2.  The pattern is a hard pattern to track for branch predictors.  This causes
    a large number of branch misses if the value is immediately checked and
    branched on. i.e. if (a != m.end()) which is a common pattern.

The patch fixes both these issues by adding the inline keyword to _M_locate
to allow the inliner to consider inlining again.

This and the other patches have been ran through serveral benchmarks where
the size, number of elements searched for and type (reference vs value) etc
were tested.

The change shows no statistical regression, but an average find improvement of
~27% and a range between ~10-60% improvements.  A selection of the results:

+-----------+--------------------+-------+----------+
| Group     | Benchmark          | Size  | % Inline |
+-----------+--------------------+-------+----------+
| Find      | unord<uint64_t     | 11274 | 53.52%   |
| Find      | unord<uint64_t     | 11254 | 47.98%   |
| Find Mult | unord<uint64_t     | 12    | 47.62%   |
| Find Mult | unord<std::string  | 12    | 44.94%   |
| Find Mult | unord<std::string  | 10    | 44.89%   |
| Find Mult | unord<uint64_t     | 11    | 40.90%   |
| Find Mult | unord<uint64_t     | 352   | 30.57%   |
| Find      | unord<uint64_t     | 351   | 28.27%   |
| Find Mult | unord<uint64_t     | 342   | 26.80%   |
| Find      | unord<std::string  | 12    | 25.66%   |
| Find Mult | unord<std::string  | 352   | 23.12%   |
| Find      | unord<std::string  | 13    | 20.36%   |
| Find Mult | unord<std::string  | 355   | 19.23%   |
| Find      | unord<std::string  | 353   | 18.59%   |
| Find      | unord<uint64_t     | 350   | 15.43%   |
| Find      | unord<std::string  | 11260 | 11.80%   |
| Find      | unord<std::string  | 352   | 11.12%   |
| Find      | unord<std::string  | 11262 | 9.97%    |
+-----------+--------------------+-------+----------+

libstdc++-v3/ChangeLog:

	* include/bits/hashtable.h: Inline _M_locate.