[cmake] Handful of small tweaks

[ChrisCummins]

This patch set:

• Removes the "subprocess" library from the bazel and CMake build configs, as it is unused as of Replace subprocess::Popen() with boost::process #398.
• Fixes a typo s/string/STRING/ that caused a harmless error to be printed.
• Removes Polly from the CMake build config. It is not used by the LLVM environment. It is only included in the Bazel build only as a workaround for a weird bug in my Bazel LLVM config.
• Strips the include/ prefix from all LLVM header includes, and the workaround in the CMake config.
• Improves the ad-hoc code for parsing LLVM sources to extract the transform pass names, fixing a couple of bugs I discovered while building against LLVM 13.0.0.1.

[ChrisCummins]

cc @sogartar (I can't add you as a reviewer)

[sogartar]

I don't have any remarks. Since you mention that parse_initialize_pass is a "throw-away" code, I was not thorough with its review.

[codecov-commenter]

Codecov Report

Merging #567 (2c2e81e) into development (22ac844) will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           development     #567   +/-   ##
============================================
  Coverage        88.06%   88.06%           
============================================
  Files              114      114           
  Lines             6693     6693           
============================================
  Hits              5894     5894           
  Misses             799      799           

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