[Sanitizer] Internal Printf string width + left-justify.

Summary:
Example:

  Printf("%-5s", "123");

should yield:

  '123  '

In case Printf's requested string field width is larger than the string
argument length, the resulting string should be padded up to the requested
width.

For the simplicity sake, implementing left-justified (right padding) only.

Reviewers: eugenis

Subscribers: kubamracek, delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D45906

llvm-svn: 330643