ThinLTO caching: reload cached file with mmap and drop heap-allocated memory buffer
ThinLTO caching: reload cached file with mmap and drop heap-allocated memory buffer

This is reducing pressure on the OS memory system, and is NFC
when not using a cache.
I measure a 10x memory consumption reduction when linking opt
with full debug info.

From: Mehdi Amini <mehdi.amini@apple.com>

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@269682 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm