Re-enable as many clang warning as possible

ICU has historically had many clang warnings disabled, but some of the
underlying causes have been fixed. One of the VC++ specific warning
disables was recently removed (crrev.com/2494793002) and this change
removes a bunch more, for clang and VC++. For some of them there are
specific icu bugs that have been fixed that allow this, and for others
I just have the evidence that on Linux and Windows icu compiles with
clang and VC++ with these changes.

BUG=21515
R=thakis@chromium.org

Review URL: https://codereview.chromium.org/2492553006 .
1 file changed