Provide -Wnull-conversion separately from -Wconversion.

Provide -Wnull-conversion separately from -Wconversion.

Like GCC, provide a NULL conversion to non-pointer conversion as a separate
flag, on by default. GCC's flag is "conversion-null" which we provide for
cross compatibility, but in the interests of consistency (with
-Wint-conversion, -Wbool-conversion, etc) the canonical Clang flag is called
-Wnull-conversion.

Patch by Lubos Lunak.
Review feedback by myself, Chandler Carruth, and Chad Rosier.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@152745 91177308-0d34-0410-b5e6-96231b3b80d8