[SR-7040] Provide a helpful diagnostic when a -D flag starts with a redundant -D

[jckarter]

Previous ID	SR-7040
Radar	None
Original Reporter	@jckarter
Type	Bug
Status	Closed
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, Driver, StarterBug
Assignee	Nonchalant (JIRA)
Priority	Medium

md5: 8973ac9d0601c5c599ecb97430f8bd6b

Issue Description:

Xcode (and possibly other IDEs) provide a UI over -D flags where you provide the flag name and -D is prepended for you. Users might reasonably add a redundant -D themselves, leading to the invalid flag `-D-DFOO` getting passed into the compiler. The driver could detect this and raise a specific error message alerting the user to their mistake.

[jckarter]

Possible implementation strategy: when we validate the names of build configuration flags, before raising the "conditional compilation flags must be valid Swift identifiers" error, check whether the flag name begins with "-D" and provide a specific diagnostic "did you provide a redundant '-D' in your build settings?"