Don't leave debug locations for constants sitting on the builder inde…

…finitely.

Because constants are currently emitted *before* the prologue, leaving the
debug location on the IRBuilder spills onto other instructions in the prologue
and messes up both line numbers as well as the point LLVM chooses to be the
prologue end.

Example LLVM IR (irrelevant IR elided):
Before:

define internal { i64, i64 } @_ZN3tmp3Foo18var_return_opt_try17he02116165b0fc08cE(ptr align 8 %self) !dbg !347 {
start:
  %self.dbg.spill = alloca [8 x i8], align 8
  %_0 = alloca [16 x i8], align 8
  %residual.dbg.spill = alloca [0 x i8], align 1
    #dbg_declare(ptr %residual.dbg.spill, !353, !DIExpression(), !357)
  store ptr %self, ptr %self.dbg.spill, align 8, !dbg !357
    #dbg_declare(ptr %self.dbg.spill, !350, !DIExpression(), !358)

After:

define internal { i64, i64 } @_ZN3tmp3Foo18var_return_opt_try17h00b17d08874ddd90E(ptr align 8 %self) !dbg !347 {
start:
  %self.dbg.spill = alloca [8 x i8], align 8
  %_0 = alloca [16 x i8], align 8
  %residual.dbg.spill = alloca [0 x i8], align 1
    #dbg_declare(ptr %residual.dbg.spill, !353, !DIExpression(), !357)
  store ptr %self, ptr %self.dbg.spill, align 8
    #dbg_declare(ptr %self.dbg.spill, !350, !DIExpression(), !358)

Note in particular how !357 from %residual.dbg.spill's dbg_declare no longer
falls through onto the store to %self.dbg.spill. This fixes argument values
at entry when the constant is a ZST (e.g. <Option as Try>::Residual). This
fixes rust-lang#130003 (but note that it does *not* fix issues with argument values and
non-ZST constants, which emit their own stores that have debug info on them,
like rust-lang#128945).

src/debuginfo.rs

@@ -48,6 +48,10 @@ impl<'a, 'gcc, 'tcx> DebugInfoBuilderMethods for Builder<'a, 'gcc, 'tcx> {
     fn set_dbg_loc(&mut self, dbg_loc: Self::DILocation) {
         self.location = Some(dbg_loc);
     }
+
+    fn clear_dbg_loc(&mut self) {
+        self.location = None;
+    }
 }
 
 /// Generate the `debug_context` in an MIR Body.