[Driver] Do a PATH lookup if needed when using -no-canonical-prefixes

When -no-canonical-prefixes option is used and argv0 contains only
a program name, we need to do a PATH lookup to get an executable path,
otherwise the return value won't be a valid path and any subsequent
uses of it (e.g. when invoking -cc1) will fail with an error.

This patch fixes PR9576.

Differential Revision: https://reviews.llvm.org/D34290

llvm-svn: 305600