tint: Remove ConversionFailure::kTooSmall

Nothing errors on this. Simplifies code.
Also: Disable MSVC warnings about constant overflow.

Fixed: tint:1564
Change-Id: I5bb2c2ebb89966f5b3f7cbcd73672e110b1b98cd
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/91622
Reviewed-by: David Neto <dneto@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

test/tint/bug/tint/1564.wgsl

@@ -0,0 +1,3 @@
+fn foo() {
+  let b = 1e-40f;
+}

test/tint/bug/tint/1564.wgsl.expected.glsl

@@ -0,0 +1,10 @@
+#version 310 es
+
+layout(local_size_x = 1, local_size_y = 1, local_size_z = 1) in;
+void unused_entry_point() {
+  return;
+}
+void foo() {
+  float b = 9.9999461e-41f;
+}
+

test/tint/bug/tint/1564.wgsl.expected.hlsl

@@ -0,0 +1,8 @@
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
+}
+
+void foo() {
+  const float b = 9.9999461e-41f;
+}

test/tint/bug/tint/1564.wgsl.expected.msl

@@ -0,0 +1,7 @@
+#include <metal_stdlib>
+
+using namespace metal;
+void foo() {
+  float const b = 9.9999461e-41f;
+}
+

test/tint/bug/tint/1564.wgsl.expected.spvasm

@@ -0,0 +1,23 @@
+; SPIR-V
+; Version: 1.3
+; Generator: Google Tint Compiler; 0
+; Bound: 9
+; Schema: 0
+               OpCapability Shader
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint GLCompute %unused_entry_point "unused_entry_point"
+               OpExecutionMode %unused_entry_point LocalSize 1 1 1
+               OpName %unused_entry_point "unused_entry_point"
+               OpName %foo "foo"
+       %void = OpTypeVoid
+          %1 = OpTypeFunction %void
+      %float = OpTypeFloat 32
+%float_0x1_16c2pn133 = OpConstant %float 0x1.16c2p-133
+%unused_entry_point = OpFunction %void None %1
+          %4 = OpLabel
+               OpReturn
+               OpFunctionEnd
+        %foo = OpFunction %void None %1
+          %6 = OpLabel
+               OpReturn
+               OpFunctionEnd

test/tint/bug/tint/1564.wgsl.expected.wgsl

@@ -0,0 +1,3 @@
+fn foo() {
+  let b = 0x1.16c2p-133;
+}