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[ADT] CoalescingBitVector: Avoid initial heap allocation, NFC

Authored by vsk on Mar 19 2020, 4:21 PM.

Description

[ADT] CoalescingBitVector: Avoid initial heap allocation, NFC

Avoid making a heap allocation when constructing a CoalescingBitVector.

This reduces time spent in LiveDebugValues when compiling sqlite3 by
700ms (0.5% of the total User Time).

rdar://60046261

Differential Revision: https://reviews.llvm.org/D76465