[clang][depscan] Support prefix mappings when deciding modules that come from "stable" directories

[cyndyishida]

To support compiler caching, clang may strip out common prefixes from
paths like the sysroot. When this happens, include both the initial
input paths that would get mapped away and the resulting mapped prefix
for determining module dependencies that come from stable directories.

This is to handle all of the ways paths may be compared. For example,
input paths that represent included headers will contain an absolute
paths for handling indirections like when vfsoverlays are used,
but also when sysroot's from CompilerInstance's are mapped.

[cyndyishida]

0003c84 is included for context but is under review against upstream.

The benefit of special handling of modules that come from stable directories is defined away when compiler caching is enabled. So, keeping this check is mostly to ensure the checks still hold when the scanner has to deal with prefix-mappings.
Some observations I'd like to confirm:

1. Whether compiler caching is on, include trees are generated, and prefixes are mapped, are independent options. I'm not sure of the reason why, e.g. if this is for testing purposes but otherwise these should all work together.
2. clang_tool is some test wrapper over clang which doesn't seem relevant still. I replaced it in tests that complained about mismatching resource directories, but if there aren't concerns I can update all the tests using it.

[jansvoboda11]

This makes sense to me, but I'd like @benlangmuir to take a look too.

[benlangmuir]

Whether compiler caching is on, include trees are generated, and prefixes are mapped, are independent options. I'm not sure of the reason why, e.g. if this is for testing purposes but otherwise these should all work together.

They're related, but somewhat independent concepts.

• Caching can be done with either include-tree or cas-fs. This is mainly historical and to maintain test coverage for the cas-fs mechanism at this point, as we only support include-tree outside tests.
• Prefix mapping is optional. If you don't prefix map you can still do caching, but the input paths need to match in order to get hits.

[cyndyishida]

@swift-ci please test LLVM

[cyndyishida]

PR failures look unrelated and/or already known

[cyndyishida]

@swift-ci please test LLVM