cgen: fix regression with .nodecl consts (#785)

## Summary

Fix a regression where the C code generator ignored the `.nodecl`
pragma for constants.

## Details

Apart from the fix, the single-use `genConstSetup` is inlined into
`useConst`. Since the loc for constants is setup in `genConstDefinition`
(which is always called before `useConst`), the `fillLoc` call is not
added to `useConst` (this is also a step towards `fillLoc` only being
used in procedures responsible for definitions).

In addition, a test for the current behaviour of imported `.nodecl`

compiler/backend/ccgexprs.nim

@@ -2136,14 +2136,9 @@ proc fillConstLoc(m: BModule, sym: PSym) =
   if sym.loc.k == locNone:
     fillLoc(sym.loc, locData, sym.ast, mangleName(m, sym), OnStatic)
 
-proc genConstSetup(m: BModule; sym: PSym): bool =
-  useHeader(m, sym)
-  fillConstLoc(m, sym)
-
-  result = lfNoDecl notin sym.loc.flags
-
 proc useConst*(m: BModule; sym: PSym) =
-  if not genConstSetup(m, sym):
+  useHeader(m, sym)
+  if lfNoDecl in sym.loc.flags:
     return
 
   assert(sym.loc.r != "", $sym.name.s & $sym.itemId)
@@ -2156,11 +2151,12 @@ proc useConst*(m: BModule; sym: PSym) =
     m.s[cfsData].add(headerDecl)
 
 proc genConstDefinition*(q: BModule; sym: PSym) =
-  let p = newProc(nil, q)
-  fillConstLoc(q, sym)
-  q.s[cfsData].addf("N_LIB_PRIVATE NIM_CONST $1 $2 = $3;$n",
-      [getTypeDesc(q, sym.typ), sym.loc.r,
-      genBracedInit(p, sym.ast, isConst = true, sym.typ)])
+  fillConstLoc(q, sym) # all consts need a valid loc
+  if lfNoDecl notin sym.loc.flags:
+    let p = newProc(nil, q)
+    q.s[cfsData].addf("N_LIB_PRIVATE NIM_CONST $1 $2 = $3;$n",
+        [getTypeDesc(q, sym.typ), sym.loc.r,
+        genBracedInit(p, sym.ast, isConst = true, sym.typ)])
 
 proc expr(p: BProc, n: PNode, d: var TLoc) =
   when defined(nimCompilerStacktraceHints):

tests/lang/s05_pragmas/s01_interop/t04_imported_constants.nim

@@ -0,0 +1,23 @@
+discard """
+  targets: "c js"
+  description: '''
+    A |NimSkull| constant can act as an alias for an external, constant
+    entity
+  '''
+"""
+
+type Obj = object # something complex that isn't inlined
+  val: int
+
+const
+  constA {.exportc.} = Obj(val: 2)
+  constB {.importc: "constA", nodecl.} = Obj(val: 1)
+
+static:
+  # in a compile-time context, the constant represents the value as is
+  # specified by the constant's initializer expression
+  doAssert constB == Obj(val: 1)
+
+# in a run-time context, the constant acts as an alias for the imported
+# entity
+doAssert constB == Obj(val: 2)