[SR-13487] SwiftPM: Allow customizing optimization level

[benlangmuir]

Previous ID	SR-13487
Radar	rdar://problem/68172740
Original Reporter	@benlangmuir
Type	Bug

Environment

5.3, Xcode 12

Additional Detail from JIRA

| | | |------------------|-----------------| |Votes | 0 | |Component/s | Package Manager | |Labels | Bug | |Assignee | None | |Priority | Medium | md5: b1aca7b2908f3273e1e306847e76442d

Issue Description:

Currently swift build -c release uses -O for swift and -O2 for C, and building a package in Xcode uses -O for swift and -Os for C.

We should be able to customize the optimization level of release builds.

Swift supports:

-Onone, -O, -Osize, -Ounchecked
-whole-module-optimization, -no-whole-module-optimization

Clang supports:

-O0, -O1, -O2, -O3, -Ofast, -Os, -Oz, -Og
-flto, -flto=full, -flto=thin

[benlangmuir]

@swift-ci create