[DLCov 1/5] Add CMake option for enhanced debug line coverage tracking (#107278)

This is part of a series of patches that tries to improve DILocation bug
detection in Debugify. This first patch adds the necessary CMake flag to
LLVM and a variable defined by that flag to LLVM's config header, allowing
the next patch to track information without affecting normal builds.

This series of patches adds a "DebugLoc coverage tracking" feature, that
inserts conditionally-compiled tracking information into DebugLocs (and
by extension, to Instructions), which is used by Debugify to provide
more accurate and detailed coverage reports. When enabled, this features
tracks whether and why we have intentionally dropped a DebugLoc,
allowing Debugify to ignore false positives. An optional additional
feature allows also storing a stack trace of the point where a DebugLoc
was unintentionally dropped/not generated, which is used to make fixing
detected errors significantly easier. The goal of these features is to
provide useful tools for developers to fix existing DebugLoc errors and
allow reliable detection of regressions by either manual inspection or
an automated script.

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