Command line option for set default value for spec constant
Format:
```
--set-spec-constant-default-value "<spec id A>:<default value A> <spec id
B>:<default value B> ..."
```
Example:
shader: `test.vert`
```
layout(constant_id = 100) const int myint = 10;
layout(constant_id = 101) const int myuint = 100;
layout(constant_id = 200) const float myfloat = 1.25;
layout(constant_id = 201) const double mydouble = 2.34;
void main() {}
```
command line:
```
spirv-opt --set-spec-const-default-value "100:12 101:200 200:1.2323
201:1.2345" test.vert -o output.spv
```
output:
```
...
OpDecorate %7 SpecId 100
OpDecorate %8 SpecId 101
OpDecorate %10 SpecId 200
OpDecorate %12 SpecId 201
%void = OpTypeVoid
%3 = OpTypeFunction %void
%int = OpTypeInt 32 1
%7 = OpSpecConstant %int 12
%8 = OpSpecConstant %int 200
%float = OpTypeFloat 32
%10 = OpSpecConstant %float 1.23232
%double = OpTypeFloat 64
%12 = OpSpecConstant %double 2.34232
...
```