Add build-clang/ to .gitignore #39

Job	Run time
Code Quality Checks	58s
Performance Regression Check	1m 22s
Build & Test (clang-15, Debug)	58s
Build & Test (clang-15, Release)	55s
Build & Test (gcc-13, Debug)	45s
Build & Test (gcc-13, Release)	52s
Sanitizer Tests (ASAN + UBSAN)	1m 31s
Build Summary	2s
	7m 23s

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