Allow -profile-guided-section-prefix more than once

Summary:
At present, `-profile-guided-section-prefix` is a `cl::Optional` option, which means it demands to be passed exactly zero or one times.  Our build system makes it pretty tricky to guarantee this.  We often accidentally pass the flag more than once (but always with the same "false" value) which results in an error, after which compilation fails:

```
clang (LLVM option parsing): for the -profile-guided-section-prefix option: may only occur zero or one times!
```

While we work on improving our build system, it also seems reasonable just to allow `-profile-guided-section-prefix` to be passed more than once, by to `cl::ZeroOrMore`.  Quoting [[ http://llvm.org/docs/CommandLine.html#controlling-the-number-of-occurrences-required-and-allowed | the documentation ]]:

> The cl::ZeroOrMore modifier ... indicates that your program will allow the option to be specified zero or more times.
> ...
> If an option is specified multiple times for an option of the cl::opt class, only the last value will be retained.

Reviewers: danielcdh

Reviewed By: danielcdh

Subscribers: twoh, david2050, llvm-commits

Differential Revision: https://reviews.llvm.org/D34219

llvm-svn: 305413