Handle interactions between reserved identifier and user-defined suffixes

According to https://eel.is/c++draft/over.literal

> double operator""_Bq(long double);  // OK: does not use the reserved identifier _­Bq ([lex.name])
> double operator"" _Bq(long double); // ill-formed, no diagnostic required: uses the reserved identifier _­Bq ([lex.name])

Obey that rule by keeping track of the operator literal name status wrt. leading whitespace.

Fix: https://bugs.llvm.org/show_bug.cgi?id=50644

Differential Revision: https://reviews.llvm.org/D104299