Properly report location of shadowed variable in warning

[andrevidela]

When warning about shadowed variables, the warning report the name of the type, rather than the location of the shadowing variable

Steps to Reproduce

The test idris/warning/warning001 shows this problem:

a1 : Char
a1 = 'a'

a2 : Char
a2 = 'a'

whyNot : a1 = a2
whyNot = ?hole

Expected Behavior

Warning: We are about to implicitly bind the following lowercase names.
You may be unintentionally shadowing the associated global definitions:
  a1 is shadowing Main.a1
  a2 is shadowing Main.a2

Issue621:8:1--8:7
 4 | 
 5 | a2 : Char
 6 | a2 = 'a'
 7 | 
 8 | whyNot : a1 = a2
              ^^^^^^^

Observed Behavior

Warning: We are about to implicitly bind the following lowercase names.
You may be unintentionally shadowing the associated global definitions:
  a1 is shadowing Main.a1
  a2 is shadowing Main.a2

Issue621:8:1--8:7
 4 | 
 5 | a2 : Char
 6 | a2 = 'a'
 7 | 
 8 | whyNot : a1 = a2
     ^^^^^^