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[ThinLTO] Launch importing backends in parallel threads from gold plugin

Description

[ThinLTO] Launch importing backends in parallel threads from gold plugin

Summary:
Launch ThinLTO backends (LTO and codegen pipelines with importing) in
parallel using a ThreadPool, after creating the combined index.
The number of threads is controlled by the existing -jobs gold plugin
option, or the hardware concurrency if not specified.

The old behavior of exiting after creating the combined index can be
invoked via a new thinlto-index-only plugin option.

This commit involves just the ThinLTO-specific pieces of D15390, the NFC
and other restructuring pieces were committed independently:

r262677: Add hardware_concurrency interface to llvm::thread (NFC)
r262719: Change split code gen to use ThreadPool
r262721: Refactor gold-plugin codegen to prepare for ThinLTO threads (NFC)

Reviewers: pcc, joker.eph, rafael

Subscribers: rafael, davidxl, llvm-commits, joker.eph

Differential Revision: http://reviews.llvm.org/D15390