Optimized the 'checkPathForIllegalChars' function.

When profiling the F# 3.0 compiler, I found the 'checkPathForIllegalChars' function was called frequently enough to show up as a performance hot-spot.

I've re-implemented the function so it uses a faster and more-efficient means of determining whether a given path contains any illegal characters. It doesn't make a huge difference in the overall runtime of the compiler, but this version seems to be fast enough to avoid showing up as a performance hot-spot when profiling now.