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>

Review URL: https://codereview.chromium.org/345643002
2 files changed