[CIR] Upstream initial attribute support (#121069)
Upstream several ClangIR-specific MLIR attributes, in particular
attributes for integer, floating-point, and null pointer constants.
These are the first ClangIR attributes to be upstreamed, so
infrastructure changes are included, such as the table-gen file
`CIRAttrs.td`.
Attributes can be used as the initial values for global variables. The
existing automated test global-var-simple.cpp includes initial values
for some of the global variables in the test.
