[BOLT] Evaluate and speedup discoverFileObjects

Operations with symbols such as name lookup are expensive and some binaries have millions of symbols, so it makes sense to try to reduce the symbol-related overheads.

Measure how much the following cost and refactor/eliminate:

1. Checks for symbol name being equal to __asan_init and __llvm_coverage_mapping for every single symbol: https://github.com/llvm/llvm-project/blob/a1423ba4278775472523fed074de6dbdfd01898a/bolt/lib/Rewrite/RewriteInstance.cpp#L824-L837
2. FileName set for every local function symbol (with O(1M) for large binaries) while we can instead use file symbol lookup with O(log N_Syms) cost and FileSyms memory cost https://github.com/llvm/llvm-project/pull/89088
3. Merge iteration over two loops over symbols into one: https://github.com/llvm/llvm-project/blob/a1423ba4278775472523fed074de6dbdfd01898a/bolt/lib/Rewrite/RewriteInstance.cpp#L824 and https://github.com/llvm/llvm-project/blob/a1423ba4278775472523fed074de6dbdfd01898a/bolt/lib/Rewrite/RewriteInstance.cpp#L876