Add some UI tests for -l modifier parsing

Author: Zalathar

tests/ui/native-library-link-flags/modifiers-bad.blank.stderr

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+error: invalid linking modifier syntax, expected '+' or '-' prefix before one of: bundle, verbatim, whole-archive, as-needed
+

tests/ui/native-library-link-flags/modifiers-bad.no-prefix.stderr

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+error: invalid linking modifier syntax, expected '+' or '-' prefix before one of: bundle, verbatim, whole-archive, as-needed
+

tests/ui/native-library-link-flags/modifiers-bad.prefix-only.stderr

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+error: unknown linking modifier ``, expected one of: bundle, verbatim, whole-archive, as-needed
+

tests/ui/native-library-link-flags/modifiers-bad.rs

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+//@ edition: 2021
+//@ revisions: blank no-prefix prefix-only unknown
+
+//@[blank] compile-flags: -l static:=foo
+//@[no-prefix] compile-flags: -l static:bundle=foo
+//@[prefix-only] compile-flags: -l static:+=foo
+//@[unknown] compile-flags: -l static:+ferris=foo
+
+// Tests various illegal values for the "modifier" part of an `-l` flag.
+
+fn main() {}

tests/ui/native-library-link-flags/modifiers-bad.unknown.stderr

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+error: unknown linking modifier `ferris`, expected one of: bundle, verbatim, whole-archive, as-needed
+