Handle trailing spaces on "settings set" command more correctly

Summary:
Currently we have some settings which treat "\ " on settings set commands specially. E.g., it is
a valid way of specifying an argument of " " to a target. However, this fails if "\ " is the last
argument as CommandObjectSettingsSet strips trailing whitespace. This resulted in a surprising
argument of "\" to the target.

This patch disables the training whitespace removal at a global
level. Instead, for each argument type we locally determine whether whitespace stripping makes
sense. Currently, I strip whitespace for all simple object type except of regex and
format-string, with the rationale that these two object types do their own complex parsing and we
want to interfere with them as least as possible. Specifically, stripping the whitespace of a
regex "\ " will result in a (surprising?) error "trailing backslash". Furthermore, the default
value of dissasembly-format setting already contains a trailing space and there is no way for the
user to type this in manually if we strip whitespace.

Reviewers: clayborg, zturner

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D7592

llvm-svn: 229382