[llvm] [llvm-driver] Add driver alias for c++filt (PR #92803)

[Fangrui Song via llvm-commits]

MaskRay wrote:

FTR: gcov-tool might be used for GCC PGO/AutoFDO. The tool is different from llvm-profdata even if they both support `xxx merge xxx yyy -o zzz`

`gcc -fprofile-generate -c a.c b.c && gcc -fprofile-generate a.o b.o -o a; ./a; gcov-tool merge . . -o dir`; gcov-tool creates a directory.

`clang -fprofile-generate -c a.c b.c && fclang -fprofile-generate a.o b.o -o a; llvm-profdata merge dir1 dir2 -o file`; llvm-profdata creates a file.

https://github.com/llvm/llvm-project/pull/92803