hwasan: Rename source files from cc to cpp

As discussed elsewhere: LLVM uses cpp as its C++ source extension; the
sanitizers should too. This updates files in hwasan.

Patch generated by

    for f in lib/hwasan/*.cc ; do svn mv $f ${f%.cc}.cpp; done

followed by

    for f in lib/hwasan/*.cpp ; do sed -i '' -e '1s/\.cc -/.cpp /' $f; done

CMakeLists.txt updated manually.

Differential Revision: https://reviews.llvm.org/D58620

llvm-svn: 354989