[clangd] Minor refactor of CanonicalIncludes::addSystemHeadersMapping.

Before commit b3a991df SystemHeaderMap used to be a vector.

Commit b3a991df changed it into a map, but neglected to remove
duplicate keys (e.g. "bits/typesizes.h", "include/stdint.h", etc.).

To prevent confusion, remove all duplicates, build HeaderMapping
one pair at a time and assert() that no duplicates are found.

Change by Paul Pluzhnikov (ppluzhnikov)!

Reviewed By: ilya-biryukov

Differential Revision: https://reviews.llvm.org/D125742