block literal irgen: several improvements on naming block

literal helper functions. All helper functions (global
and locals) use block_invoke as their prefix. Local literal
helper names are prefixed by their enclosing mangled function
names. Blocks in non-local initializers (e.g. a global variable 
or a C++11 field) are prefixed by their mangled variable name. 
The descriminator number added to end of the name starts off 
with blank (for first block) and _<N> (for the N+2-th block).

llvm-svn: 159206