Various cleanups:

- Remove tabs
- Standardize use of space around ( and ).
- Consolidate the ConstantPlaceHolder class
- Rename two methods to be more meaningful (ParseType, ParseTypes)
- Correct indentation of blocks
- Add documentation
- Convert input dependent asserts to error(...) so it throws instead.
Provide placeholder implementations of read_float and read_double that
still read in platform-specific endianess. When I figure out how to do
this without knowing the endianess of the platform, it will get implemented
correctly.

llvm-svn: 14765