Improve process launch comments for Windows

The existing comment about over-allocating the command line was incorrect.  The
contents of the command line may be changed, but it's not necessary to over
allocate.  The changes will be limited to the existing contents of the string
(e.g., by replacing spaces with L'\0' to tokenize the command line).

Also added a comment explaining a possible cause of failure to save the next
programmer some time when they try to debug a 64-bit process from a 32-bit
LLDB.

llvm-svn: 355121