[MC] Fix floating-point literal lexing.

This patch has three related fixes to improve float literal lexing:

1. Make AsmLexer::LexDigit handle floats without a decimal point more
   consistently.
2. Make AsmLexer::LexFloatLiteral print an error for floats which are
   apparently missing an "e".
3. Make APFloat::convertFromString use binutils-compatible exponent
   parsing.

Together, this fixes some cases where a float would be incorrectly
rejected, fixes some cases where the compiler would crash, and improves
diagnostics in some cases.

Patch by Brandon Jones.

Differential Revision: https://reviews.llvm.org/D57321

llvm-svn: 357214