[flang] Block construct

A block construct is an execution control construct that supports
declaration scopes contained within a parent subprogram scope or another
block scope. (blocks may be nested.) This is implemented by applying
basic scope processing to the block level.

Name uniquing/mangling is extended to support this. The term "block" is
heavily overloaded in Fortran standards. Prior name uniquing used tag `B`
for common block objects. Existing tag choices were modified to free up `B`
for block construct entities, and `C` for common blocks, and resolve
additional issues with other tags. The "old tag -> new tag" changes can
be summarized as:

     -> B  -- block construct -> new
  B  -> C  -- common block
  C  -> YI -- intrinsic type descriptor; not currently generated
  CT -> Y  -- nonintrinsic type descriptor; not currently generated
  G  -> N  -- namelist group
  L  ->    -- block data; not needed -> deleted

Existing name uniquing components consist of a tag followed by a name
from user source ...