[ASan] Create new files asan_report.{h,cc} as a preparation for refactoring of ASan error reporting code.
Currently ASan reports many kinds of errors, and the code that actually prints error messages can
be found inside allocator, OS-specific files, interceptors code etc.

An example of maintenance troubles this situation causes:
There is currently an ASan interface function that registers
callback which should take the char buffer with error report printed by ASan.
This function is now broken, as one has to insert callback calls to all the places in
ASan code where the error reports are printed, surprisingly it is not only
"__asan_report_error" function...

llvm-svn: 161568