Cherry-pick: ICU-20250 faster MutableCodePointTrie.build(): use a hashtable to find equal blocks in earlier parts of the compacted data & index arrays

Upstream commit: https://github.com/unicode-org/icu/commit/59006770ed0a11d197cf39cbee92e4c96ab00ed8

Improve the class initialization of DateTimePatternGenerator$FormatParser by ~70%.
The time taken to initialize the class is comparable or even better than
ICU 62.

Bug: 118748230
Test: CtsIcuTestCases
Change-Id: I250945d190c6d1900caf7fda4f69db9eab2fbe8f
3 files changed