[clangd] Collect definitions when indexing.

Within a TU:
 - as now, collect a declaration from the first occurrence of a symbol
   (taking clang's canonical declaration)
 - when we first see a definition occurrence, copy the symbol and add it
Across TUs/sources:
 - mergeSymbol in Merge.h is responsible for combining matching Symbols.
   This covers dynamic/static merges and cross-TU merges in the static index.
 - it prefers declarations from Symbols that have a definition.
 - GlobalSymbolBuilderMain is modified to use mergeSymbol as a reduce step.
Random cleanups (can be pulled out):
 - SymbolFromYAML -> SymbolsFromYAML, new singular SymbolFromYAML added
 - avoid uninit'd SymbolLocations. Add an idiomatic way to check "absent".
 - CanonicalDeclaration (as well as Definition) are mapped as optional in YAML.
 - added operator<< for Symbol & SymbolLocation, for debugging

Reviewers: ioeric, hokein

Subscribers: klimek, ilya-biryukov, jkorous-apple, cfe-commits

Differential Revision: https://reviews.llvm.org/D42942

llvm-svn: 324735