clang/win: For 64-bit builds, enable warnings-as-errors and stop falling back to CL.

Clang can now build all of chrome's translation units in 64-bit.  Remove
/fallback so that the bot will turn red if this regresses.

Also turn on warnings-as-errors, so that warnings also turn the build red
(instead of just causing a fallback as before).

Also remove a superfluous 'RuntimeTypeInfo': 'false': This is already the
default for non-clang builds too.  (And for the few places where we do need
RTTI -- icu -- clang has been able to emit it for months now.)

BUG=82385
R=hans@chromium.org

Review URL: https://codereview.chromium.org/921633004

Cr-Commit-Position: refs/heads/master@{#316141}
1 file changed