RFR: 8331187: Optimize MethodTypeDesc and ClassDesc.ofDescriptor for primitive types

[Claes Redestad]

This PR makes ClassDesc.ofDescriptor return the shared constant for primitive descriptor strings ("I" etc..), and leverages this further by refactoring `MethodTypeDescImpl.ofDescriptor` to avoid the intermediate substring for primitives. 

Microbenchmarks results look good with expected speedups and allocation reductions any time a primitive is part of the descriptor string, and a non-significant cost otherwise:


Name                                                             (descString) Cnt     Base     Error       Test     Error   Unit  Change
ClassDescFactories.ofDescriptor                            Ljava/lang/Object;   6   13,941 ±   1,643     14,071 ±   1,333  ns/op   0,99x (p = 0,681 )
  :gc.alloc.rate.norm                                                               16,000 ±   0,000     16,000 ±   0,000   B/op   1,00x (p = 0,617 )
ClassDescFactories.ofDescriptor                                             V   6    9,212 ±   1,045      1,405 ±   0,049  ns/op   6,55x (p = 0,000*)
  :gc.alloc.rate.norm                                                               48,000 ±   0,000      0,000 ±   0,000   B/op   0,00x (p = 0,000*)
ClassDescFactories.ofDescriptor                                             I   6    9,009 ±   0,035      1,431 ±   0,192  ns/op   6,30x (p = 0,000*)
  :gc.alloc.rate.norm                                                               48,000 ±   0,000      0,000 ±   0,000   B/op   0,00x (p = 0,000*)
MethodTypeDescFactories.ofDescriptor  (Ljava/lang/Object;Ljava/lang/String;)I   6  182,050 ±   4,333    141,644 ±   2,685  ns/op   1,29x (p = 0,000*)
  :gc.alloc.rate.norm                                                              360,001 ±   0,000    264,001 ±   0,000   B/op   0,73x (p = 0,000*)
MethodTypeDescFactories.ofDescriptor                                      ()V   6   17,169 ±   2,008      9,915 ±   0,018  ns/op   1,73x (p = 0,000*)
  :gc.alloc.rate.norm                                                              120,000 ±   0,000    104,000 ±   0,000   B/op   0,87x (p = 0,000*)
MethodTypeDescFactories.ofDescriptor ([IJLjava/lang/String;Z)Ljava/util/List;   6  270,372 ±   3,624    217,050 ±   3,170  ns/op   1,25x (p = 0,000*)
  :gc.alloc.rate.norm                                                              520,002 ±   0,000    328,001 ±   0,000   B/op   0,63x (p = 0,000*)
MethodTypeDescFactories.ofDescriptor                    ()[Ljava/lang/String;   6   35,036 ±   0,351     36,010 ±   3,882  ns/op   0,97x (p = 0,146 )
  :gc.alloc.rate.norm                                                              104,000 ±   0,000    104,000 ±   0,000   B/op   1,00x (p = 0,140 )
  * = significant

-------------

Commit messages:
 - Restructure micros
 - Update ConstantUtils.parseMethodDescriptor
 - Merge in ClassDescriptor micro

Changes: https://git.openjdk.org/jdk/pull/18971/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18971&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331187
  Stats: 126 lines in 6 files changed: 88 ins; 9 del; 29 mod
  Patch: https://git.openjdk.org/jdk/pull/18971.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18971/head:pull/18971

PR: https://git.openjdk.org/jdk/pull/18971