[sanitizer][win] Change cmdline check to allow double backslashs

When `llvm-symbolizer.exe` is on the PATH in an entry containing two
consecutive backslashes, sanitizers will try to launch llvm-symbolizer
with its absolute path containing these consecutive backslashes. This
fails a sanity check in `sanitizer_symbolizer_win.cpp`.

According to the documentation of `CommandLineToArgvW` [1] and a MS blog
post [2], backslashes in general, regardless of how many of them in a
row, do not have any special effect, unless when immediately followed by
a double quote.

There already exists a check that fails when the command line arguments
contains double quote, therefore the check for double backslashes can
simply be removed.

[1]: https://learn.microsoft.com/en-us/windows/win32/api/shellapi/nf-shellapi-commandlinetoargvw
[2]: https://learn.microsoft.com/en-us/archive/blogs/twistylittlepassagesallalike/everyone-quotes-command-line-arguments-the-wrong-way

Differential Revision: https://reviews.llvm.org/D146621