.calypso_cache corruption in certain use cases

[timotheecour]

to reproduce:

mkdir temp && cd temp
$ldc2 -cpp-verbosediags -cpp-args -std=c++11 -run $git_clone_D/D/Calypso/tests/calypso/basics.d

# ok

cp $git_clone_D/D/Calypso/tests/calypso/basics.d .
ldc2 -cpp-verbosediags -cpp-args -std=c++11 -cpp-args -I$git_clone_D/D/Calypso/tests/calypso -run basics.d

Calypso/tests/calypso/basics.cpp:6:8: error: redefinition of 'testDoubleVar'
double testDoubleVar = 99.99;
       ^
Calypso/tests/calypso/basics.cpp:6:8: note: previous definition is here
double testDoubleVar = 99.99;

also, error message is non-sensical since previous definition location==redefinition location

WORKAROUND: rm -rf .calypso_cache but would be nice to either have better diagnostic error messages or fix this altogether or tell user to clear cache