[Parser] Parse annotations, including source map comments (#6345)

Parse annotations using the standards-track `(@annotation ...)` format as well
as the `;;@ source-map:0:1` format. Have the lexer implicitly collect
annotations while it skips whitespace and add lexer APIs to access the
annotations since the last token was parsed. Collect annotations before parsing
each instruction and pass the annotations explicitly to the parser and parser
context functions for instructions. Add an API to `IRBuilder` to set a debug
location to be attached to the next visited or created instruction and use it
from the parser.

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