[SR-3660] Add support for custom build configuration

[swift-ci]

Previous ID	SR-3660
Radar	rdar://problem/40205598
Original Reporter	jaybuff (JIRA User)
Type	New Feature

Additional Detail from JIRA

| | | |------------------|-----------------| |Votes | 2 | |Component/s | Package Manager | |Labels | New Feature | |Assignee | None | |Priority | Medium | md5: b376729fade38ce68c114991e74bd9fe

is duplicated by:

• SR-5004 Swift build with -whole-module-optimization fails

relates to:

• SR-3948 Support Build Settings

Issue Description:

After reading https://github.com/apple/swift-package-manager/blob/master/Documentation/Reference.md#build-configurations I realized I want a configuration that gives me:

• -g: Generate debug information.

• -O: Compile with optimizations.

• -whole-module-optimization: Optimize input files (per module) together instead of individually.

For builds that are going to be put on a server.

[aciidgh]

We don't yet support custom build configurations but I think it should be possible to generate debugging symbols in release mode by passing -g via Xswiftc i.e. $ swift build -Xswiftc -g -c release
Can you try that and report if that doesn't work for some reason?

[aciidgh]

@swift-ci create