remove misleading diagnostics for values exceeding bitfield width

[mycoboco]

struct foo { int a: 3; };

void f(void)
{
    struct foo s = { 9223372036854775808 };
}

foo.c:5:22: warning - integer constant is so large that it is unsinged
      struct foo s = { 9223372036854775808 };
                       ^~~~~~~~~~~~~~~~~~~
foo.c:5:22: warning - overflow in converting constant expression from `unsigned long long' to `long long int'
      struct foo s = { 9223372036854775808 };
                       ^~~~~~~~~~~~~~~~~~~
foo.c:5:22: warning - initializer exceeds bit-field
      struct foo s = { 9223372036854775808 };
                       ^~~~~~~~~~~~~~~~~~~
foo.c:5:16: warning - local `s' set but not used [-Wextra]
      struct foo s = { 9223372036854775808 };
                 ^

[mycoboco]

Difficult to completely remove diagnostics from enode_cast() because:

• simple muting is inappropriate when a large fp initializer is given for bit-fields;
• changing the diagnostic in question to, say, overflow in implicit conversion not mentioning types would help, but require enode_cast() to tell if the diagnostic has been issued; and
• modifying enode_cast() to accept bitfield info would help, but changing every invocation of it should be a burden.

[mycoboco]

Seems not bad to separate a bit-field overflow check into two, one to check if an initializer fits in the storage unit and the other to check if fits in a bit-field if the first diagnostic message shows the type of the storage unit type correctly.