Merging r358547:

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r358547 | ruiu | 2019-04-16 19:12:47 -0700 (Tue, 16 Apr 2019) | 9 lines

Fix a crash bug caused by a nested call of parallelForEach.

parallelForEach is not reentrant. We use parallelForEach to call
each section's writeTo(), so calling the same function within writeTo()
is not safe.

Fixes https://bugs.llvm.org/show_bug.cgi?id=41508

Differential Revision: https://reviews.llvm.org/D60757
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llvm-svn: 360791