Where is the C API reference for the MSP430? CCS version
These are defined in the includes. For CCS, the default is the TI created msp430.h, which then attempts to include the specific file for your target device through if defines. That file (i.e. msp4430g2553.h) is which houses the specific references like P1OUT or P1DIR matching to the specific register address as defined in your Target Device's Datasheet and Family guide. For the most part, the names should match the Family Guide, and are very consistent between devices.

From the CCS User Guide:


  Use .h files to simplify code development.
  
  CCS is supplied with files for each device that define the device registers and the bit names. Using these files is recommended and can greatly simplify the task of developing a program. To include the .h file corresponding to the target device, add the line #include  for C  and .cdecls C,LIST,"msp430xyyy" for assembly code, where xyyy specifies the MSP430 part number.


Here is an online copy of the msp430g2553.h from CCS. Very well commented and easy to read.

IAR, as well as having the msp430.h versions, also includes io430.h, which is created by IAR and has a few differences or additional features. It also acts the same way of including the specific target device's header.