[clang][lex][minimizer] Avoid treating path separators as comments

The minimizer strips out single-line comments (introduced by `//`). This sequence of characters can also appear in `#include` or `#import` directives where they play the role of path separators. We already avoid stripping this character sequence for `#include` but not for `#import` (which has the same semantics). This patch makes it so `#import <A//A.h>` is not affected by minimization. Previously, we would incorrectly reduce it into `#import <A`.

Reviewed By: arphaman

Differential Revision: https://reviews.llvm.org/D119226