cmSourceFile: Check if a file is GENERATED first in the full path computation

In `cmSourceFile::FindFullPath` check first if the file is GENERATED before
aborting on `FindFullPathFailed`.  This allows recomputation of the full path
when the GENERATED property was set after the file path was computed with an
error.
Status Job ID Name Coverage
  External
passed cdash

passed cmake-dash3win7-windows-release-ninja

passed cmake-hythloth-linux-debug-makefiles+sphinx

passed cmake-hythloth-linux-release-lint-iwyu-tidy

passed cmake-moonshade-osx-debug-ninja

passed cmake-moonshade-osx-release-makefiles

passed cmake-terfin-windows-release-vs2017

passed ghostflow-check-master

passed ghostflow-stager

passed cmake-dash3win7-windows-release-ninja

passed cmake-dash3win7-windows-release-ninja

00:24:45

passed cmake-dash3win7-windows-release-ninja

passed cmake-dash3win7-windows-release-ninja

passed cmake-hythloth-linux-debug-makefiles+sphinx

passed cmake-hythloth-linux-debug-makefiles+sphinx

00:55:35

passed cmake-hythloth-linux-debug-makefiles+sphinx

passed cmake-hythloth-linux-debug-makefiles+sphinx

passed cmake-hythloth-linux-release-lint-iwyu-tidy

passed cmake-hythloth-linux-release-lint-iwyu-tidy

00:05:41

passed cmake-hythloth-linux-release-lint-iwyu-tidy

passed cmake-hythloth-linux-release-lint-iwyu-tidy

passed cmake-moonshade-osx-debug-ninja

00:10:32

passed cmake-moonshade-osx-debug-ninja

passed cmake-moonshade-osx-debug-ninja

passed cmake-moonshade-osx-debug-ninja

passed cmake-moonshade-osx-release-makefiles

passed cmake-moonshade-osx-release-makefiles

passed cmake-moonshade-osx-release-makefiles

passed cmake-moonshade-osx-release-makefiles

00:12:37

passed cmake-terfin-windows-release-vs2017

passed cmake-terfin-windows-release-vs2017

passed cmake-terfin-windows-release-vs2017

passed cmake-terfin-windows-release-vs2017

00:46:03

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager