Swiftc port of https://github.com/apple/llvm-project/pull/4207.
This introduces a new flag, `-file-prefix-map` which can be used
instead of the existing `-debug-prefix-map` and `-coverage-prefix-map`
flags, and also remaps paths in index information currently.
Several tests related to indexing system modules were taking a considerable
amount of time (100+ seconds in the worst case) indexing the standard library.
This adds a frontend option to skip it and updates those tests to pass it.
