clang-format: [JS] Optionally re-quote string literals.

Turns "foo" into 'foo' (or vice versa, depending on configuration).
This makes it more convenient to follow the Google JavaScript style
guide:
https://google.github.io/styleguide/javascriptguide.xml?showone=Strings#Strings

This functionality is behind the option "JavaScriptQuotes", which can be:

  * "leave" (no re-quoting)
  * "single" (change to single quotes)
  * "double" (change to double quotes)

This also changes single quoted JavaScript string literals to be treated
as tok::string_literal, not tok::char_literal, which fixes two unrelated
tests.

Patch by Martin Probst. Thank you.

llvm-svn: 262534