Cherry-pick DwarfExpression bugfix.#362

Merged

adrian-prantl merged 2 commits intoswiftlang:apple/stable/20191106from

adrian-prantl:57294211-1

Nov 21, 2019

Commits on Nov 21, 2019

Fix an offset underflow bug in DwarfExpression when describing small values with subregisters
adrian-prantl
committed
Reduce the number of iterations in testcase. (NFC)
adrian-prantl
committed