[llvm-rc] Implement escape sequences in .rc files.

This allows the escape sequences (\a, \n, \r, \t, \\, \x[0-9a-f]*,
\[0-7]*, "") to appear in .rc scripts. These are parsed and output in
the same way as it's done in original MS implementation.

The way these sequences are processed depends on the type of the
resource it resides in, and on whether the user declared the string to
be "wide" or "narrow".

I tried to maintain the maximum compatibility with the original tool
(and fail in some erroneous situations that are accepted by .rc).
However, there are some (extremely rare) cases where Microsoft tool
outputs nonsense. I found it infeasible to detect such casses.

Patch by Marek Sokolowski

Differential Revision: https://reviews.llvm.org/D38426

llvm-svn: 315118