[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 th

[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.

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[CIR] floating-point, pointer, and function types (#120484)

Upstream ClangIR support for `void` type, floating-point types, pointer
types, and function types.

Floating-point support is missing t

[CIR] floating-point, pointer, and function types (#120484)

Upstream ClangIR support for `void` type, floating-point types, pointer
types, and function types.

Floating-point support is missing the IBM double-double format, because
that hasn't been implemented in the incubator project yet.

Pointer types do not yet support address spaces.

Function type support includes only the return type and the parameter
types. The many other properties and attributes of function types will
be upstreamed later.

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[CIR] Integral types; simple global variables (#118743)

Add integral types to ClangIR. These are the first ClangIR types, so the
change includes some infrastructure for managing ClangIR types.

S

[CIR] Integral types; simple global variables (#118743)

Add integral types to ClangIR. These are the first ClangIR types, so the
change includes some infrastructure for managing ClangIR types.

So that the integral types can be used somewhere, generate ClangIR for
global variables using the new `cir.global` op. As with the current
support for functions, global variables are just a stub at the moment.
The only properties that global variables have are a name and a type.

Add a new ClangIR code gen test global-var-simple.cpp, which defines
global variables with most of the integral types.

(Part of upstreaming the ClangIR incubator project into LLVM.)

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