[WebAssembly] Add -emscripten-cxx-exceptions-whitelist option

This patch adds -emscripten-cxx-exceptions-whitelist option to
WebAssemblyLowerEmscriptenExceptions pass. This options is the list of
function names in which Emscripten-style exception handling is enabled.
This is to support emscripten's EXCEPTION_CATCHING_WHITELIST which
exists because of the performance impact of emscripten's non-zero-cost
EH method.

Patch by Heejin Ahn

Differential Revision: https://reviews.llvm.org/D23292

llvm-svn: 278171