[Lexer] Speedup LexTokenInternal

Only reset "NeedsCleaning" flag in case of re-entrant call.
Do not needlessly blank IdentifierInfo. This information will be set
once the token type is picked.

This yields a nice 1% speedup when pre-processing sqlite amalgamation
through:

valgrind --tool=callgrind ./bin/clang -E sqlite3.c -o/dev/null

Differential Revision: https://reviews.llvm.org/D137960