Feat(compiler_base_error): Add methods to the DiagnosticHandler that emits Diagnostic as string.

[zong-zhe]

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

• N
• Y

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

compiler_base_error.

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 method "emit_all_diags_into_string" for 'DiagnosticHandler' to emit all diagnostics into strings.
add method "emit_nth_diag_into_string" for 'DiagnosticHandler' to emit one diagnostics into string.

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 4414298391

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 31 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-26.7%) to 63.619%

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Files with Coverage Reduction	New Missed Lines	%
compiler_base/session/src/lib.rs	31	44.64%

Totals
Change from base Build 4413770963:	-26.7%
Covered Lines:	22415
Relevant Lines:	35233

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💛 - Coveralls

[Peefy]

LGTM