<rdar://problem/14035604>

Fixing an issue where formats would not propagate from parents to children in all cases
Details follow:
an SBValue has children and those are fetched along with their values
Now, one calls SBValue::SetFormat() on the parent
Technically, the format choices should propagate onto the children (see ValueObject::GetFormat())
But if the children values are already fetched, they won't notice the format change and won't update themselves
This commit fixes that by making ValueObject::GetValueAsCString() check if any format change intervened from the previous call to the current one
A test case is also added



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@183030 91177308-0d34-0410-b5e6-96231b3b80d8