Fix JS_WIDESTRING to work with clang-cl

Fix JS_WIDESTRING to work with clang-cl

MSVC lexes L#macro_arg as a single wide string literal token, but
Clang and other C/C++ compliant lexers do not.  There was already
a workaround to use implicit string concatenation for GCC, but there's
a simpler solution of token pasting the L onto the stringized macro
argument with 'L###macro_arg'.  This works with Clang, GCC, and MSVC.

R=jun_fang@foxitsoftware.com, jam@chromium.org
BUG=82385
Original patch by Reid Kleckner <rnk@chromium.org>;

Committed: https://pdfium.googlesource.com/pdfium/+/63412bf

[jam, 2014-06-18 22:19:41]

Redirecting to Bo or Jun

[jun_fang, 2014-06-19 00:42:20]

On 2014/06/18 22:19:41, jam wrote:
> Redirecting to Bo or Jun

LGTM

[jam, 2014-06-19 15:07:42]

On 2014/06/19 00:42:20, jun_fang wrote:
> On 2014/06/18 22:19:41, jam wrote:
> > Redirecting to Bo or Jun
> 
> LGTM

Thanks, committed in https://pdfium.googlesource.com/pdfium/+/63412bf