[cmake] fix malformed path with double slash in CMakeLists.txt

[x12301450]

Remove the trailing slash of ROOT_LLVM_MAIN_SRC_DIR in CMakeLists.txt.

This PR fixes is #21093

[github-actions]

Test Results

    22 files      22 suites   3d 17h 44m 3s ⏱️
 3 776 tests  3 775 ✅ 0 💤 1 ❌
75 992 runs  75 991 ✅ 0 💤 1 ❌

For more details on these failures, see this check.

Results for commit 4c62a81.

[Copilot]

Pull request overview

This PR fixes a path formatting issue in the CMake configuration by removing a trailing slash from the ROOT_LLVM_MAIN_SRC_DIR variable definition. This prevents the creation of malformed paths with double slashes when the variable is concatenated with other path components that begin with a leading slash.

Changes:

• Removed trailing slash from ROOT_LLVM_MAIN_SRC_DIR path definition

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[guitargeek]

I'm curious: what's the benefit of this change? Can CMake not deal with double slashes? Normally that doesn't make the path invalid.