Improve buildflag header guard generation to handle unsupported chars.

Buildflag header generator uses directory structure to create header
guards. Any characters that cannot be used as a part of guard identifier
should be replaced, but prior this change not all invalid characters
were replaced.

Currently Chromium does not have issues with invalid guards in buildflag
headers, but other Chromium-based projects may encounter this issue. It
seems reasonable to fix this globally. The more generalized approach
covers all possible invalid chars.

Bug: 1313910
Change-Id: I543f9d1286defb7c3b94a010800d3e79fb1dcca2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3571751
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Auto-Submit: Aleksey Khoroshilov <akhoroshilov@brave.com>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#990336}