swift234

Reapply "Remove the locks from getGenericMetadata to accelerate Debug builds."

This commits also reduces the size of the metadata cache by allocating the
map dynamically,

There are no regressions and many perf wins:
NestedLoop     1.96x
Life           1.92x
CaptureProp    1.84x
Ary            1.67x
Ary2           1.60x
ArraySubscript 1.57x
ArrayLiteral   1.54x
StdlibSort     1.52x
Havlak         1.51x
TwoSum         1.49x
DollarFilter   1.37x
PrimeNum       1.35x
NBody          1.30x
RangeAssignmen 1.29x
DollarFunction 1.28x
Walsh          1.27x
Memset         1.27x
Histogram      1.27x
RIPEMD         1.27x

Swift SVN r23358