Support pre-loaded buffers in SourceLoader

[povik]

Hi,

please see if this is acceptable. I have a use-case where I want to use Driver to get standard slang command line parsing, but I want to inject a piece of input which doesn't originate from a file.

[codecov]

Codecov Report

Attention: Patch coverage is 53.33333% with 7 lines in your changes missing coverage. Please review.

Project coverage is 94.79%. Comparing base (f4e8ec4) to head (f19a57f).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
source/driver/SourceLoader.cpp	54.54%	5 Missing ⚠️
include/slang/driver/SourceLoader.h	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1164      +/-   ##
==========================================
- Coverage   94.80%   94.79%   -0.02%     
==========================================
  Files         191      193       +2     
  Lines       49873    50003     +130     
==========================================
+ Hits        47280    47398     +118     
- Misses       2593     2605      +12     

Files with missing lines	Coverage Δ
include/slang/driver/SourceLoader.h	77.77% <50.00%> (-22.23%)	⬇️
source/driver/SourceLoader.cpp	91.97% <54.54%> (-1.26%)	⬇️

... and 12 files with indirect coverage changes

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[MikePopoloski]

This approach seems fine to me, just one comment to fix.

[povik]

I have addressed the comment.