[SanitizerBinaryMetadata] Make constructors/destructors hidden

By switching them to external with default visibility, DSOs may not call
their own constructor/destructor. This is incorrect, because they pass
different parameters.

Fix it by marking the ctors/dtors as external linkage but with hidden
visibility.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D143611