block literal irgen: several improvements on naming block

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).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@159206 91177308-0d34-0410-b5e6-96231b3b80d8