[lldb][NFCI] Pre-allocate storage in ObjCLanguage::MethodName::GetFullNameWithoutCategory

The size of a full ObjC MethodName can vary somewhat, but it is
computable ahead of time.

Using a reasonably sized ObjC application, this actually improves the
time it takes to initialize symbol indexes for ObjC names ever so
slightly. Additionally, I found that the variability in time also was
improved considerably.

Differential Revision: https://reviews.llvm.org/D150914