[cmake] Strip quotes in try_compile_only

After r372209, the compile command can end up including an argument with
quotes in it, e.g.

  -fprofile-instr-use="/foo/bar.profdata"

when invoking the compiler with execute_process, the compiler ends up
getting that argument with quotes and all, and fails to open the file.

This all seems horribly broken, but one way of working around it is to
simply strip the quotes from the string here. If they were there to
protect a path that's got spaces in it, that wasn't going to work
anyway because the string is later split by spaces.

llvm-svn: 372312