removed emptyString and its remaining usage

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In my limited local tests I did not see any measurable performance impact. So posting to see if it has any effect in the CI.

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The __GLIBC__ check only worked because of the <string>. The backtrace() function is not available on all platforms but I have not figured out how to properly check for it. So just enable it unconditionally of that and wait for some feedback. We probably should split up these preprocessor checks and make them base on the feature/function we use.

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With the current changes there is at 11% increase in Tokenizer::simplifyTypedefCpp():

  633,270,864 ( 1.00%)  build/tokenize.cpp:Tokenizer::simplifyTypedefCpp() [/home/runner/work/cppcheck/cppcheck/cppcheck]

  704,783,980 ( 1.11%)  build/tokenize.cpp:Tokenizer::simplifyTypedefCpp() [/home/runner/work/cppcheck/cppcheck/cppcheck]

But that still doesn't account for the total increase:

63,276,354,832 (100.0%)  PROGRAM TOTALS

63,413,640,429 (100.0%)  PROGRAM TOTALS