fix(material/chips): chip input not showing placeholder (#30664)

The chip input is inheriting some styles from `MatInput` which were causing it not to show its placeholder unless it's focused.

These changes override the inherited styles to show it correctly.

Fixes #16380.

Author: crisbeto

src/material/chips/chip-set.scss

@@ -1,3 +1,5 @@
+@use '../core/style/vendor-prefixes';
+
 // Ensures that the internal chip container spans the entire outer container width, if the
 // outer container width is customized. This is used by some wrapper components in g3.
 .mat-mdc-chip-set {
@@ -63,4 +65,19 @@ input.mat-mdc-chip-input {
     margin-left: 0;
     margin-right: 8px;
   }
+
+  // The form field hides the placeholder by default,
+  // but we actually want to show it if there are chips.
+  .mat-mdc-form-field:not(.mat-form-field-hide-placeholder) & {
+    @include vendor-prefixes.input-placeholder {
+      opacity: 1;
+    }
+  }
+
+  // The margin isn't necessary if the input is placed next to the chip set,
+  // because it'll be on the next line.
+  .mat-mdc-chip-set + & {
+    margin-left: 0;
+    margin-right: 0;
+  }
 }