This patch changes the `Platform::LocateXXX` to return a map from
`FileSpec` to `LoadScriptFromSymFile` enum.
This is needed for https://github.com/llvm/llvm-project/pull/188722,
where I intend to set `LoadScriptFromSymFile` per-module.
By default the `Platform::LocateXXX` set the value to whatever the
target's current `target.load-script-from-symbol-file` is set to. In
https://github.com/llvm/llvm-project/pull/188722 we'll allow overriding
this per-target setting on a per-module basis.
Drive-by:
* Added logging when we fail to load a script.
89dec12692