[Interpreter] Temporarily prefer /usr/lib/swift for Swift dylibs loaded by the interpreter

[mikeash]

This is a cherry-pick of some 5.0 work plus a bit more, that's temporarily needed for 5.1. That need will hopefully go away soon, at which point I'll redo this to prefer the libraries next to the swift binary, and use /usr/lib/swift as a fallback.

rdar://problem/49610198

[mikeash]

@swift-ci please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - df0dd3a72f1d7a7fca96e5e0cdc79855347b86e7

[shahmishal]

@swift-ci test macOS

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - df0dd3a72f1d7a7fca96e5e0cdc79855347b86e7

[mikeash]

@swift-ci please test linux

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - df0dd3a72f1d7a7fca96e5e0cdc79855347b86e7

[shahmishal]

@swift-ci test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - df0dd3a72f1d7a7fca96e5e0cdc79855347b86e7

[shahmishal]

00:53:37.045 ******************** TEST 'Swift(macosx-x86_64) :: Driver/environment-mac.swift' FAILED ********************
00:53:37.045 Script:
00:53:37.045 --
00:53:37.045 : 'RUN: at line 3';   env SDKROOT= '/Users/buildnode/jenkins/workspace/swift-PR-osx/branch-swift-5.1-branch/buildbot_incremental/swift-macosx-x86_64/bin/swift' -module-cache-path '/Users/buildnode/jenkins/workspace/swift-PR-osx/branch-swift-5.1-branch/buildbot_incremental/swift-macosx-x86_64/swift-test-results/x86_64-apple-macosx10.9/clang-module-cache' -swift-version 4  -driver-use-frontend-path /Users/buildnode/jenkins/workspace/swift-PR-osx/branch-swift-5.1-branch/swift/test/Driver/Inputs/print-var.sh /Users/buildnode/jenkins/workspace/swift-PR-osx/branch-swift-5.1-branch/swift/test/Driver/environment-mac.swift DYLD_LIBRARY_PATH | '/usr/bin/python' '/Users/buildnode/jenkins/workspace/swift-PR-osx/branch-swift-5.1-branch/swift/utils/PathSanitizingFileCheck' --sanitize BUILD_DIR='/Users/buildnode/jenkins/workspace/swift-PR-osx/branch-swift-5.1-branch/buildbot_incremental/swift-macosx-x86_64' --sanitize SOURCE_DIR='/Users/buildnode/jenkins/workspace/swift-PR-osx/branch-swift-5.1-branch/swift' --use-filecheck '/Users/buildnode/jenkins/workspace/swift-PR-osx/branch-swift-5.1-branch/buildbot_incremental/llvm-macosx-x86_64/bin/FileCheck' /Users/buildnode/jenkins/workspace/swift-PR-osx/branch-swift-5.1-branch/swift/test/Driver/environment-mac.swift
00:53:37.045 --
00:53:37.045 Exit Code: 1
00:53:37.045 
00:53:37.045 Command Output (stderr):
00:53:37.045 --
00:53:37.045 
/Users/buildnode/jenkins/workspace/swift-PR-osx/branch-swift-5.1-branch/swift/test/Driver/environment-mac.swift:5:11: error: CHECK: expected string not found in input
00:53:37.045 // CHECK: {{^/usr/lib/swift:}}
00:53:37.045           ^
00:53:37.045 <stdin>:2:1: note: scanning from here
00:53:37.045 
00:53:37.045 ^
00:53:37.045 
00:53:37.045 --
00:53:37.045 
00:53:37.045 ********************

[mikeash]

@swift-ci please smoke test

[mikeash]

@swift-ci please smoke test

[mikeash]

@swift-ci please smoke test

[mikeash]

@swift-ci please smoke test OS X platform

[mikeash]

@swift-ci please test OS X platform

[mikeash]

@swift-ci please smoke test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - df0dd3a72f1d7a7fca96e5e0cdc79855347b86e7

[mikeash]

@swift-ci please test OS X platform

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - f7f8a3bf0cb876d7ebbc2689db376b3e97fb8e4c

[mikeash]

@swift-ci please smoke test and merge

[mikeash]

@swift-ci please smoke test and merge

[mikeash]

@swift-ci please smoke test and merge

[mikeash]

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - df0dd3a72f1d7a7fca96e5e0cdc79855347b86e7

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - d8c5806f4555fe7c6c3d1bf4e4a95939fcd26329