Bump LLVM from 14 to 15

[jumormt]

1. use llvm 15 in build.sh
2. Fix deprecated API getpointerelementtype
3. build extapi.bc with opaque ptr disabled for now

[codecov]

Codecov Report

Merging #1242 (160f1e5) into master (bf3ed56) will decrease coverage by 0.02%.
Report is 1 commits behind head on master.
The diff coverage is 33.33%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1242      +/-   ##
==========================================
- Coverage   64.44%   64.42%   -0.02%     
==========================================
  Files         223      223              
  Lines       23713    23711       -2     
==========================================
- Hits        15281    15276       -5     
- Misses       8432     8435       +3     

Files	Coverage Δ
svf-llvm/include/SVF-LLVM/LLVMUtil.h	73.68% <ø> (ø)
svf-llvm/tools/Example/svf-ex.cpp	36.66% <100.00%> (ø)
svf/lib/MemoryModel/AccessPath.cpp	3.65% <0.00%> (ø)

... and 3 files with indirect coverage changes