Fix Rust test cases

PR rust/25025 notes that some Rust test cases fail.

Debugging gdb revealed that the Rust compiler emits different linkage
names that demangle to the same result.  Enabling complaints when
reading the test case is enough to show it:

    During symbol reading: Computed physname <generics::identity<f64>> does not match demangled <generics::identity> (from linkage <_ZN8generics8identity17h8540b320af6656d6E>) - DIE at 0x424 [in module /home/tromey/gdb/build/gdb/testsuite/outputs/gdb.rust/generics/generics]
    During symbol reading: Computed physname <generics::identity<u32>> does not match demangled <generics::identity> (from linkage <_ZN8generics8identity17hae302fad0c33bd7dE>) - DIE at 0x459 [in module /home/tromey/gdb/build/gdb/testsuite/outputs/gdb.rust/generics/generics]
    ...

This patch changes the DWARF reader to prefer the computed physname,
rather than the output of the demangler, for Rust.  This fixes the
bug.

gdb/ChangeLog
2020-04-24  Tom Tromey  <[email protected]>

	PR rust/25025:
	* dwarf2/read.c (dwarf2_physname): Do not demangle for Rust.

Author: tromey

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2020-04-24  Tom Tromey  <[email protected]>
+
+	PR rust/25025:
+	* dwarf2/read.c (dwarf2_physname): Do not demangle for Rust.
+
 2020-04-24  Tom Tromey  <[email protected]>
 
 	PR symtab/12707:

gdb/dwarf2/read.c

@@ -10318,7 +10318,8 @@ dwarf2_physname (const char *name, struct die_info *die, struct dwarf2_cu *cu)
   if (!die_needs_namespace (die, cu))
     return dwarf2_compute_name (name, die, cu, 1);
 
-  mangled = dw2_linkage_name (die, cu);
+  if (cu->language != language_rust)
+    mangled = dw2_linkage_name (die, cu);
 
   /* DW_AT_linkage_name is missing in some cases - depend on what GDB
      has computed.  */