SourceLocation.init should enforce valid arguments

[stmontgomery]

This modifies SourceLocation.init() to add preconditions which enforce that its arguments are valid, matching preconditions in the corresponding properties.

Modifications:

• Add preconditions to the initializer which match those in didSet of the corresponding properties.
• Add precondition DocC symbol documentation where relevant.
• Adjusted existing tests which were previously invalid (although not particularly problematic) but were not failing.

Result:

Newly-added exit tests now pass.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[stmontgomery]

@swift-ci please test

[stmontgomery]

I just moved this down without modification, to consolidate several exit tests together

[grynspan]

We should consider adding comments to all the precondition() calls since they don't do source capture, e.g.:

precondition(line > 0, "SourceLocation.line must be greater than 0 (was \(line))")

[stmontgomery]

@swift-ci please test

[stmontgomery]

@swift-ci please test