[extract_symbols.py] Export unique symbols

As described in a comment on D140637, PluginInlineOrderTest.NoInlineFoo
will fail with:

InlineOrderPlugin.so: undefined symbol: _ZN4llvm13AllAnalysesOnINS_6ModuleEE6SetKeyE

The symbol is unique in libLLVMCore and ends up local in AnalysisTests.
Fix this by exporting all unique symbols found in libraries.

AnalysisTests.symbols change in line count: 12464 -> 12499

Differential Revision: https://reviews.llvm.org/D146731