Feat(compiler-base): add util functions to span.

[zong-zhe]

1. Does this PR affect any open issues?(Y/N) and add issue references (e.g. "fix #123", "re #123".):

• N
• Y

issue #115

2. What is the scope of this PR (e.g. component or file name):

compiler_base/span/src/lib.rs.
compiler_base/span/src/span.rs.

3. Provide a description of the PR(e.g. more details, effects, motivations or doc link):

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Other

add util function span_to_filename_string() to get filename from SourceMap by Span.
add util function new_byte_pos() to create rustc_span::BytePos.

4. Are there any breaking changes?(Y/N) and describe the breaking changes(e.g. more details, motivations or doc link):

• N
• Y

5. Are there test cases for these changes?(Y/N) select and add more details, references or doc links:

• Unit test
• Integration test
• Benchmark (add benchmark stats below)
• Manual test (add detailed scripts or steps below)
• Other

6. Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[coveralls]

Pull Request Test Coverage Report for Build 3005565496

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.5%) to 59.917%

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Totals
Change from base Build 3000577825:	-0.5%
Covered Lines:	22762
Relevant Lines:	37989

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💛 - Coveralls

[ldxdl]

LGTM

[Peefy]

LGTM