[runtime] Mark some readonly unicode functions as being pure.
[runtime] Mark some readonly unicode functions as being pure.

These come in two categories of functions:

1. Comparison.
  - swift_stdlib_unicode_compare_utf16_utf16
  - swift_stdlib_unicode_compare_utf8_utf16
  - swift_stdlib_unicode_compare_utf8_utf8

2. Hashing.
  - swift_stdlib_unicode_hash
  - swift_stdlib_unicode_hash_ascii

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