[SR-15087] C/C++ Flags needed by package maintainers in SPM

[swift-ci]

Previous ID	SR-15087
Radar	rdar://problem/82133353
Original Reporter	kneekey23 (JIRA User)
Type	Improvement

Additional Detail from JIRA

| | | |------------------|-----------------| |Votes | 1 | |Component/s | | |Labels | Improvement | |Assignee | @abertelrud | |Priority | Medium | md5: 6218395650a1eb0b2374c85eb97b0154

Issue Description:

As a package author, at a minimum I want to be able to have control of the flags listed in https://bugs.swift.org/browse/SR-15086 as well as:

• `-W` and its various permutations

• `-D` - need to be able to define macros

• Flags mentioned in https://bugs.swift.org/browse/SR-15086 are crucial to just producing decent callstacks and debugging info

For debugging purposes, it would be extremely helpful to be able to link in the sanitizers via

• `-fsanitize=(address|thread)`

[typesanitizer]

@swift-ci create