Rename Appveyor install helper script.

I'm really lazy and the old name for the helper script
killed 2 letter tab completion for the include directory.
This patch renames it to avoid that problem and enable
lazyness.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@302619 91177308-0d34-0410-b5e6-96231b3b80d8

appveyor.yml

@@ -36,7 +36,7 @@ install:
   ############################################################################
   # All external dependencies are installed in C:\projects\deps
   ############################################################################
-  - call "%APPVEYOR_BUILD_FOLDER%\\install-appveyor-reqs.cmd"
+  - call "%APPVEYOR_BUILD_FOLDER%\\appveyor-reqs-install.cmd"
 
 before_build:
   - if DEFINED MSVC_SETUP_PATH call "%MSVC_SETUP_PATH%" %MSVC_SETUP_ARG%