SE-0387: add --toolset option to build-related subcommands

[MaxDesiatov]

Motivation:

The only feature proposed in SE-0387 that remains unimplemented is the --toolset CLI option:

We propose that users also should be able to pass --toolset <path_to_toolset.json> option to swift build, swift test, and swift run.

We'd like to allow using multiple toolset files at once. This way users can "assemble" toolchains on the fly out of tools that in certain scenarios may even come from different vendors. A toolset file can have an arbitrary name, and each file should be passed with a separate --toolset option, i.e. swift build --toolset t1.json --toolset t2.json.

All of the properties related to names of the tools are optional, which allows merging configuration from multiple
toolset files. For example, consider toolset1.json:

{
 "schemaVersion": "1.0",
 "swiftCompiler": {
   "path": "/usr/bin/swiftc",
   "extraCLIOptions": ["-Xfrontend", "-enable-cxx-interop"]
 },
 "cCompiler": {
   "path": "/usr/bin/clang",
   "extraCLIOptions": ["-pedantic"]
 }
}

and toolset2.json:

{
 "schemaVersion": "1.0",
 "swiftCompiler": {
   "path": "/custom/swiftc"
 }
}

With multiple --toolset options, passing both of those files will merge them into a single configuration. Tools passed in subsequent --toolset options will shadow tools from previous options with the same names. That is,
swift build --toolset toolset1.json --toolset toolset2.json will build with /custom/swiftc and no extra flags, as specified in toolset2.json, but /usr/bin/clang -pedantic from toolset1.json will still be used.

Tools not specified in any of the supplied toolset files will be looked up in existing implied search paths that are used without toolsets, even when rootPath is present. We'd like toolsets to be explicit in this regard: if a tool would like to participate in toolset path lookups, it must provide either a relative or an absolute path in a toolset.

Tools that don't have path property but have extraCLIOptions present will append options from that property to a tool with the same name specified in a preceding toolset file. If no other toolset files were provided, these options will be appended to the default tool invocation.

Modifications:

Added toolsetPaths on LocationOptions, which passes new customToolsets argument on SwiftSDK.deriveTargetSwiftSDK. Added corresponding tests.

Result:

New --toolset option is able to accept toolset JSON files.

[MaxDesiatov]

@swift-ci test

[MaxDesiatov]

@swift-ci test

[MaxDesiatov]

@swift-ci test

[MaxDesiatov]

@swift-ci test

[MaxDesiatov]

@swift-ci test windows

[dschaefer2]

Looks good.

[MaxDesiatov]

@swift-ci test

[MaxDesiatov]

@swift-ci test windows