DXASSERT condition for failed template specialization needs amending

[tcorringham]

Description
A large codebase that is successfully compiled in many projects sometimes triggers a DXASSERT when specializing a template type - with no reason being apparent.

Steps to Reproduce
// bug.hlsl - a very artificial example that illustrates the issue
float a;
#include "a.h" // include file doesn't exist - this should produce an fatal error diagnostic
void b() {};
int3 c(int X)
{
return X.xxx; // assert triggered if include file a.h doesn't exist, compilation succeeds if it does
}
// dxc -T lib_6_8 bug.hlsl

Actual Behavior
An investigation revealed the DXASSERT was only triggered if there was some error detected earlier in the compilation - e.g. an include file is not found.
Clang suppresses template specialization if a fatal error has been reported in order to avoid the risk of a cascade of secondary error diagnostics from a failed specialization.
The DXASSERTs only check for a failed specialization, without allowing for the failure being due to an earlier unrelated fatal error - this has the unintended result of hiding the fatal error, and providing no indication of what the problem is.

The DXASSERTs should not trigger if a fatal error has been registered.

Environment

• DXC version :libdxcompiler.so: 1.8(dev;4588-f6d17595)
• Host Operating System : Any