This will be more correct if a user file is named prelude.carbon, and
reduces the explorer runtime by about 10% by removing a string
comparison from the check for whether trace output is enabled.
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Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
Replacing direct raw_string_ostream uses. I figure the wrapper should be used more consistently.
There are still remaining raw_string_ostream uses that weren't compatible -- I'm continuing to look at those, but felt it was cleaner to have this on its own.
This allows us to combine multiple Errors together without repeating the prefix
information. Also fixes several cases where two "COMPILATION ERROR" prefixes
would be prepended to the same message when errors with prefixes and locations
were produced by the lexer and parser.